lifornia] 

tonal 

lity 


THE  LIBRARY 

OF 

THE  UNIVERSITY 
OF  CALIFORNIA 

LOS  ANGELES 


is  ""r  "«  last  rf'te  ». 


0  ^   >J'- 


THE  STORY 

OF    THE    EARTH'S 

ATMOSPHERE 


BY 
DOUGLAS   ARCHIBALD,  M.A. 

FELLOW  AND  SOMETIME  VICE-PRESIDENT 
OF  THE   ROYAL  METEOROLOGICAL  SOCIETY,    LONDON 


WITH     FORTY-FOUR     ILLUSTRATIONS 


NEW  YORK 
MCMXII 


COPYRIGHT,  1897,  1902, 
BY  D.  APPLETON  AND  COMPANY. 


tngmeering  & 

Mathematical 

Sciences 

Library 

IK* 

A  47 


PREFACE. 


I  HAVE  desired  in  the  present  little  work  to 
put  forward  the  main  features  of  our  knowledge 
of  the  conditions  which  prevail  in  our  atmosphere 
as  they  are  interpreted  through  the  science  of  to- 
day. The  Atmosphere,  unlike  its  solid  partner, 
contains  no  gold  or  coal  mines  with  which  to 
stimulate  scientific  research.  Its  study  has  con- 
sequently been  somewhat  neglected  until  of  late 
years,  and  is  even  now  only  just  emerging  from 
the  stage  of  myth  and  speculation  into  that  of 
fact  and  certainty. 

This  desirable  result  has  been  chiefly  attained 
by  the  disuse  of  vague  speculation  and  the  appli- 
cation of  the  known  laws  of  physics. 

I  have  therefore  written,  not  for  the  minority, 
who  vaguely  wonder  at  the  relation  of  extraordi- 
nary facts  and  pass  on,  but  for  what  I  believe  to 
be  that  much  more  numerous  section  who  are  not 
content  with  a  mere  collection  of  facts,  but  want 
to  know  the  reason  why. 

I  have  levied  largely  upon  the  original  works 
of  the  more  modern  school  of  meteorologists 
which  is  so  ably  represented  in  America,  India, 
and  Germany — and  am  under  especial  obligations 
to  those  of  Prof.  Davis  of  Harvard,  Prof.  Loomis 
5 


6   THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

of  Yale,  Mr.  Ferrel  of  Washington,  Prof.  Sprung, 
and  Prof.  Waldo. 

I  have  purposely  omitted  the  subject  of  weather 
and  descriptions  of  instruments,  and  only  briefly 
touched  upon  climate,  and  have  rather  endeav- 
oured to  show,  especially  in  the  chapter  on  Flight, 
that  the  Atmosphere  possesses  growing  uses  and 
interests  quite  apart  from,  and  in  addition  to,  its 
consideration  as  a  vehicle  of  weather. 

DOUGLAS  ARCHIBALD. 


CONTENTS. 


CHAPTER  PACK 

I.  THE  ORIGIN  AND  HEIGHT  OF  THE  ATMOSPHERE.  9 
II.   THE  NATURE  AND  COMPOSITION  OF  THE  AT- 
MOSPHERE          17 

III.  THE  PRESSURE  AND  WEIGHT  OF  THE  ATMOS- 

PHERE       25 

IV.  THE  TEMPERATURE  OF  THE  ATMOSPHERE        .  31 
V.  THE  GENERAL  CIRCULATION   OF  THE  ATMOS- 
PHERE        64 

VI.   THE  LAWS  WHICH  RULE  THE  ATMOSPHERE    .  94 
VII.   THE  DEW,  FOG,  AND  CLOUDS  OF  THE  ATMOS- 
PHERE        106 

VIII.   THE  RAIN,  SNOW,  AND  HAIL  OF  THE  ATMOS- 
PHERE        ng 

IX.  THE  CYCLONES  OF  THE  ATMOSPHERE       .        .  125 

X.  THE  SOUNDS  OF  THE  ATMOSPHERE  .        .        .  138 
XI.   THE  COLOURS  AND  OPTICAL  PHENOMENA  OF 

THE  ATMOSPHERE .  141 

XII.   WHIRLWINDS,  WATERSPOUTS,  TORNADOES,  AND 

THUNDERSTORMS  OF  THE  ATMOSPHERE        .  149 

XIII.  SUSPENSION  AND  FLIGHT  IN  THE  ATMOSPHERE  163 

XIV.  LIFE  IN  THE  ATMOSPHERE 183 


LIST   OF   ILLUSTRATIONS. 


PAGE                                                                                   PAGE 

Cumulus  Cloud 

Fig.  23  —  "  After      the 

Frontispiece 

Storm  "... 

97 

Fig.    i  —  Strato-  Cumulus 

Fig.  24  —  Diffusive  Limits 

(low)     .... 

ii 

of  the  Component  Gases 

Fig.    2  —  Strato-Cumulus 

of  the  Atmosphere 

10  1 

(high)   .... 

14 

Fig.  25  —  CirrusCloud  (var 

Fig.    3  —  Cirro-Cumulus  . 

17 

Tracto  Cirrus,  1889)    . 

H3 

Fig.    4     .... 

33 

Fig.  26     .... 

US 

Fig.    5     .... 

35 

Fig.  27     .... 

116 

Fie    6 

A.1 

Fig.  28  —  Festooned  Cumu- 

Fig    7 

*TJ 

AC 

lus        .... 

nS 

Fig.    8  —  Distribution    of 

T-D 

Fig.  29    . 

121 

Atmospheric  Tempera- 
ture   in    Latitude    for 

Fig.  30     . 
Fig.  31     . 

I23 
124 

January,  July,  and  the 

Fig.  32     . 

129 

year      .... 

47 

Fig.  33     . 

I32 

Fig.    9    .... 

53 

Fig.  34     . 

133 

Fig.  10     .... 

55 

Fig.  35     • 

J33 

Fig.  n     . 

56 

Fig.  36  —  Tornado  Funnel 

Fig.  12     . 

. 

57 

cioud  :     . 

J55 

Fig-  13     . 

67 

Fig.  37  —  Thunderstorm 

Fig.  14     . 

69 

in  Section     . 

157 

Fig.  15     . 

. 

72 

Fig.  38—  Kestrel     Hawk 

Fig.  16     . 

73 

Hovering     . 

168 

Fig.  17    . 

. 

74 

Fig.  39     ..-. 

171 

Fig.  18    . 

. 

75 

Fig.  40     .... 

175 

Fig.  19     . 

78 

Fig.  41     . 

176 

Fig.  20    . 

81 

Fig.  42     . 

173 

Fig.  21     . 
Fig.  22     . 

, 

83 
86 

Fig.  43  —  Yachting  in  Syd- 
ney Harbour 

181 

THE  STORY  OF  THE  EARTH'S 
ATMOSPHERE. 


CHAPTER  I. 

THE    ORIGIN    AND    HEIGHT    OF    THE  ATMOSPHERE, 

THE  atmosphere  of  air  in  which  we  live  and 
breathe  is  really  a  part  of  the  solid  globe  on 
which  we  stand. 

Until  we  think  of  it,  we  might  be  inclined  to 
imagine  we  were  surrounded  by  mere  space,  but 
when  we  place  our  heads  under  water  we  find  we 
can  not  live  more  than  a  few  seconds  without  in- 
haling the  same  air,  and  we  have  only  to  look  at 
our  ships  sailing,  our  windmills  rotating,  and  our 
slates  blowing  off  our  roofs  in  a  storm,  to  be  cer- 
tain that  it  is  just  as  material  as  the  solid  earth 
to  which  it  clings. 

Its  past  history,  unlike  that  of  its  more  solid 
partner,  is  not  written  in  the  unmistakable  lan- 
guage of  successive  rock  strata,  or  fossil  remains, 
and  we  can  only  infer  something  of  its  ancient 
changes  from  analogy  with  what  is  now  occurring 
in  the  sun,  and  a  knowledge  of  the  physical  his- 
tory of  the  universe. 

If  we  are  to  believe  the  "  nebular  theory," 
propounded  years  ago  by  the  great  French  astron- 
omer, La  Place,  and  which,  far  from  being  upset, 
has  rather  been  confirmed  by  recent  discovery, 
9 


10     THE    STORY  OF  THE   EARTH'S  ATMOSPHERE. 

all  existing  suns  and  planets  have  been  simply 
condensed  from  clouds  or  nebulae  of  matter  origi- 
nally scattered  through  space. 

By  the  mutual  attraction  of  their  matter 
(which  force  we  now  term  gravitation),  these 
separate  aggregations  became  highly  heated  glob- 
ular masses,  every  element  of  which  was  at  first 
in  a  state  of  fiery  gaseous  incandescence.  As 
they  gradually  cooled  and  threw  off  planetary  ex- 
crescences, these  masses  became  condensed  at  first 
into  liquid  spheres  or  suns,  surrounded  by  atmos- 
pheres of  the  lighter  and  less  condensible  gases, 
still  hot  enough  to  be  luminous.  Of  such  a  type 
is  our  own  sun. 

A  further  stage  of  cooling  took  place,  par- 
ticularly amongst  the  planetary  offspring,  during 
which  the  liquid  cooled  enough  on  its  external 
surface  to  form  a  thin  solid  crust,  beneath  which 
it  still  remained  more  or  less  liquid,  and  above 
which  enough  gases  still  remained  uncondensed 
to  form  a  thin  atmosphere,  through  which  light 
and  heat  could  penetrate,  and  yet  substantial 
enough  to  support  animal  life.  This  is  the  pres- 
ent condition  of  our  own  planet. 

We  must  not,  however,  suppose  that  this  state 
of  things  holds  on  every  other  planet.  The  rate 
at  which  such  changes  progress  is  different  for 
each  planet. 

The  planet  Jupiter  is  still  so  hot  that  it  is  be- 
lieved to  be  partly  self-luminous,  and  its  atmos- 
phere probably  contains  clouds  and  vapours  of 
substances  which  on  our  cooler  earth  have  long 
since  condensed  into  liquids  or  solids.  Through 
the  telescope  it  is  seen  to  be  covered  with  dense 
clouds,  and  most  of  its  water  probably  still  exists 
in  the  form  of  vapour  (or  water  gas),  and  not  in 


ORIGIN  AND   HEIGHT  OF  THE  ATMOSPHERE.      II 

liquid  seas  as  on  our  own  globe.  The  planet 
Mars,  on  the  other  hand,  has  so  little  water  left 
in  its  atmosphere  or  on  its  surface  that,  while 
enough  remains  to  supply  its  polar  caps  with 
snow  during  the  winter,  its  parched  equatorial 
deserts  are  believed  by  Mr.  Lowell,  of  the  Arizona 
Observatory,  and  others  who  have  made  it  a 


FIG.  i. — Strato-cumulus  (low). 

special  study,  to  be  irrigated  thence  by  the 
system  of  so-called  canals  which  intersect  its 
surface. 

Finally,  our  moon  presents  a  picture  of  the 
condition  eventually  reached  by  a  small  globe — 
viz.,  all  solid,  no  liquid,  and  no  gas  left.  There- 
fore, according  to  our  ideas,  no  life  would  be 
possible  on  the  moon.  The  liquid,  which  would 


12  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

be  chiefly  water,  has  been  absorbed  into  the  solid 
substance  of  the  moon,  while  the  last  relics  of 
the  gaseous  atmosphere,  which  it  once  must  un- 
doubtedly have  possessed,  have  been  either  ab- 
sorbed into  its  mass  or  else  diffused  into  space 
beyond  the  power  of  recall  by  gravitation. 

The  condition  of  each  globe  at  present  de- 
pends chiefly  on  the  rate  at  which  these  changes 
from  all  gas,  to  gas  and  liquid,  and  thence  to  gas, 
liquid,  and  solid,  occur — /.  <?.,  on  their  rate  of  cool- 
ing. The  larger  the  globe  the  longer  it  takes  to 
cool. 

The  final  condition,  however — viz.,  whether  a 
globe  ultimately  ceases  to  possess  a  liquid  or 
gaseous  covering,  and  becomes  like  our  moon,  or 
still  retains  an  atmosphere  and  oceans  like  our 
earth,  depends  on  the  attraction  (gravity,  as  we 
term  it)  by  which  it  holds  its  gaseous  portions  to 
it.  This,  again,  directly  depends  on  the  amount 
of -matter  it  contains,  and  therefore  again  upon 
its  size.  Thus,  our  earth  will  probably  never  lose 
its  atmosphere  altogether,  though  considerable 
quantities  of  the  lighter  gases,  such  as  hydrogen, 
have  no  doubt  already  escaped  into  space. 

The  fact,  therefore,  that  we  possess  at  the 
present  time  a  gaseous  atmosphere  of  exactly 
that  particular  degree  of  tenuity  that  suits  our 
breathing  apparatus,  remarkable  though  it  may 
seem,  is  a  direct  consequence  of  the  particular 
size  of  the  globe  on  which  we  stand. 

Back  through  the  corridors  of  time,  before  the 
earth  had  sufficiently  cooled  to  acquire  a  solid 
crust,  we  were  a  little  sun,  with  an  atmosphere  of 
hot,  turbid,  metallic  vapours  which  poured  down 
metallic  rain,  only  to  be  boiled  off  again  on 
approaching  the  heated  surface.  After  a  time, 


ORIGIN   AND    HEIGHT   OF  THE  ATMOSPHERE.      13 

however,  such  metallic  rain  would  cease  to  rise 
again,  and  remain  a  part  of  the  solidifying  earth, 
and  by  the  time  that  geologic  history  com- 
menced and  the  surface  was  cool  enough  to  ad- 
mit of  animal  and  vegetable  growth,  the  atmos- 
phere must  have  been  practically  as  clear  as  it  is 
to-day. 

In  proof  of  this  we  find  that  those  remarkable 
trilobites  or  sea-lice  of  the  Silurian  period,  which 
is  nearly  the  oldest  of  which  we  have  any  knowl- 
ledge,  were  endowed  with  organs  of  vision,  which 
shew  that  as  much  light  penetrated  the  seas  then 
as  now.  The  atmosphere,  therefore,  must  have 
been  equally  transparent.  Doubtless,  more  va- 
pour and  carbonic  acid  were  present.  Indeed, 
some  of  the  latter  has  since  been  locked  up  in  a 
solid  form  in  the  coal  measures  and  limestone 
rocks  of  subsequent  epochs. 

Continuing  our  globe  history,  there  came  a 
time  when  the  atmosphere,  after  being  heated 
mostly  from  the  still  warm  earth,  began  to  find 
its  solid  partner  no  longer  the  warm  friend  of  its 
youth,  and  found  itself  compelled  to  depend  on 
the  solar  beams,  albeit  after  they  had  travelled 
through  ninety-three  million  miles  of  space,  to 
protect  it  from  the  terrible  cold  of  space.  By 
receiving  and  entrapping  such  rays,  it  is  even  now 
enabled  to  keep  some  500°  Fahr.  warmer  than 
outside  space,  while  the  heat  which  at  present 
reaches  it  from  the  earth  is  estimated  as  being 
barely  enough  to  raise  it  -j-J^ths  of  a  degree  in 
temperature. 

The  atmosphere  of  our  planet,  therefore,  is 
our  own  individual  property,  and  in  no  sense 
part  of  a  universal  atmosphere  spread  all  over 
space.  In  fact,  if  such  a  general  atmosphere  ex- 


14  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

isted  at  all,  it  has  been  calculated  by  Dr.  Thiesen 
of  Berlin  that  our  sun  would,  by  virtue  of  its 
enormous  size — a  million  times  that  of  our  earth 
• — and  gravity,  which  is  twenty-seven  times 
greater,  attach  to  itself  a  gaseous  covering  or 
atmosphere,  which  would  be  as  dense  as  our  own, 
far  beyond  the  orbit  of  Venus.  This,  however, 
is  known  to  be  contrary  to  fact. 


FIG.  2.— Strato-cumulus  (high). 


The  sun's  atmosphere  is  not  more  than  about 
500,000  miles  deep,  while  that  of  the  earth  is  cer- 
tainly not  more  than  100  miles. 

The  height  of  our  atmosphere  has  never  been 


ORIGIN   AND    HEIGHT   OF   THE   ATMOSPHERE.      15 

measured  as  we  measure  distances  on  the  earth's 
surface,  for  the  very  simple  reason  that  we  can 
never  hope  to  reach  the  top.  Indeed,  we  should 
find  it  very  difficult  to  know  where  the  top  was, 
even  if  we  were  able  to  approach  it,  since  the  air 
would  shade  off  so  gradually  into  where  it  sud- 
denly changed  into  the  vacuum  of  space  that  we 
should  with  difficulty  discover  the  place  where 
we  could  say  "  thus  far  and  no  farther." 

We  can,  however,  arrive  at  some  knowledge 
of  the  probable  height  to  which  the  air  exists  in 
such  quantity  as  to  possess  weight  and  resistance 
by  calculation  of  the  rate  at  which  the  pressure 
of  the  atmosphere  diminishes  as  we  ascend,  and 
also  by  observation  of  the  duration  of  twilight 
and  the  heights  at  which  meteorites  (or,  as  they 
are  still  popularly  termed,  falling  stars)  are 
visible. 

Living  as  we  do  at  the  base  of  our  ocean  of 
air,  like  the  flat-fish  live  at  the  bottom  of  the 
ocean  of  water,  we  are  absurdly  ignorant  of  the 
condition  of  the  atmosphere  a  few  miles  overhead. 

The  highest  ascent  made  by  man  up  moun- 
tains is  believed  to  be  that  of  Zurbriggen  on 
Aconcaqua,  when  he  reached  about  24,000  feet, 
or  a  little  over  4  miles,  while  the  highest  in  a 
balloon  was  that  made  by  Dr.  Berson  of  Berlin, 
who  in  1894  ascended  to  a  height  of  30,000  feet. 

Some  years  ago,  in  1862,  Glaisher  and  Coxwell 
made  a  memorable  ascent  over  Wolverhampton, 
when  they  became  unconscious  at  29,000  feet, 
after  which  they  were  supposed  to  have  ascended 
for  a  short  time,  to  nearly  36,000  feet,  but  in  Dr. 
Berson's  case,  by  inhaling  oxygen  he  was  able  to 
observe  his  instruments  and  carefully  note  the 
conditions  around  him. 


1 6  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

His  thermometer  went  down  to  54  degrees 
below  zero  Fahr.,  while  the  mercury  in  his  bar- 
ometer sank  from  30  to  9  inches.  Six  miles  is 
probably  the  limit  to  which  man  will  ever  care  to 
ascend  into  the  atmosphere,  since  above  this 
height  he  can  only  survive  by  the  aid  of  artificial 
assistance.  For  permanent  habitation  it  is  found 
to  be  prejudicial  to  live  at  greater  heights  than 
15,000  feet,  so  that  it  is  only  within  a  thin  slice 
of  our  atmospheric  blanket  that  human  life  is 
lived.  Actually,  the  marvellous  complexity  of 
human  thought  and  action,  and  the  development 
of  modern  civilisation  on  this  earth,  has  taken 
place,  and  will  probably  always  remain  confined 
within  the  vertical  distance  of  a  London  shilling 
cab  fare  above  the  surface. 

Apart  from  direct  measurement,  the  pressure 
of  the  atmosphere  gives  us  some  clue  to  its  height 
as  well  as  to  its  weight.  From  the  pressure  obser- 
vations alone,  it  ought  to  disappear  somewhere 
about  38  miles,  since  at  that  height  the  mercury 
column  of  the  barometer,  which  measures  the 
weight  of  air  above,  would  tend  to  disappear. 
Observations  of  meteorites,  however,  whose  ap- 
pearance depends  upon  their  heating  to  incan- 
descence by  friction  against  a  resisting  medium, 
shew  that  some  air  exists  at  100  miles,  though 
at  such  great  altitudes  it  is  probably  in  a  con- 
dition of  extreme  rarity.  Observations  of  the 
duration  of  twilight,  which  is  due  to  reflection 
from  particles  of  dust  and  air,  gave  about  50 
miles  as  the  limit.  Practically,  therefore,  we 
may  take  50  miles  to  be  about  the  limit  up  to 
which  the  atmosphere  exists  in  a  coherent  form 
as  we  know  it  near  the  earth's  surface. 


NATURE  AND  COMPOSITION  OF  ATMOSPHERE.      17 


CHAPTER   II. 

THE    NATURE    AND    COMPOSITION    OF    THE 
ATMOSPHERE. 

To  one  of  those  superior  beings  who,  we  be- 
lieve, inhabit  the  celestial  regions,  it  must  have 
been  infinitely  pathetic  to  see  the  poor  human  mites 
on  this  planet  struggling  for  centuries  through 
the  mist  of  error  and  superstition,  until  they 
finally  discovered  one  day  the  composition  of  the 
atmosphere  in  which  they  lived.  By  the  Greeks 


FIG.  3. — Cirro-cumulus. 

the  air  was  considered  to  be  one  of  the  four  ele- 
ments, and  it  was  not  until  the  middle  of  the  last 
century  that  Priestley  discovered  that  air  was  a 


1 8  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

mixture  of  oxygen  and  nitrogen,  and  that  its 
neutral  character  was  due  to  the  blending  of  a 
most  active  element,  oxygen,  with  a  most  inactive 
element,  nitrogen. 

A  slight  difference  in  the  proportion  of  either 
element  would  be  fatal  to  life  as  we  know  it. 
With  more  oxygen  in  the  air  our  lives,  short 
enough  as  they  are,  would  be  still  more  brief,  and 
though  we  might  be  more  witty  and  brilliant,  we 
should  live  in  a  state  of  such  mental  and  physical 
intoxication  that  we  should  never  be  able  to  sit 
down  quietly  to  do  any  solid  work.  In  fact,  the 
human  race  would  be  converted  into  a  number  of 
thoughtless,  reckless,  frivolous  beings,  who  would 
probably  end  by  destroying  each  other  in  a  frenzy 
of  over-excitement.  On  the  other  hand,  too  much 
nitrogen  would  reduce  us  to  such  a  degree  of 
dulness  and  inertia  that  our  supposed  national 
characteristics  would  be  intensified  and  we  should 
become  like  a  row  of  statues  or  mummies,  with, 
out  action  or  passion,  lifeless — in  fact,  matter 
without  motion.  The  existing  proportion  there- 
fore is  decidedly  adapted  to  our  present  require- 
ments. The  average  proportion  in  which  the  two 
principal  components  of  the  atmosphere  are  found 
to  occur  is  21  of  oxygen  to  79  of  nitrogen  by  vol- 
ume, and  23  of  oxygen  to  77  of  nitrogen  by  weight. 

The  proportion  in  which  the  remaining  con- 
stituents enter  is  so  small  that  it  may  be  practi- 
cally neglected  when  we  consider  the  physical 
properties  of  the  atmosphere,  though  it  cannot 
be  neglected  when  we  regard  its  vital  and  chemi- 
cal functions.  The  other  constituents  are  car- 
bonic acid,  which  occupies  i  o  §  jnrtns  by  volume, 
traces  of  ammonia,  ozone,  and  the  recently  dis- 
covered argon. 


NATURE  AND  COMPOSITION  OF  ATMOSPHERE.      19 

Oxygen,  which  forms  one-fifth  of  the  atmos- 
phere, represents  the  active  vitalising  principle,  a 
large  proportion  of  which,  by  its  former  chemical 
union  with  certain  terrestrial  elements,  such  as 
silicon  and  aluminium,  has  solidified  into  large 
rock  masses,  by  union  with  hydrogen,  has  pro- 
duced the  liquid  ocean,  and  the  gaseous  vapour 
of  the  atmosphere,  and  which,  by  its  chemical 
union  with  carbon  through  the  tissues  of  plants 
and  animals,  develops  the  energy  which  is  mani- 
fested in  their  life  and  movements. 

Owing  to  the  fact  that  the  density  of  oxygen 
is  very  nearly  the  same  as  that  of  nitrogen,  and 
to  the  constant  mixture  which  takes  place,  the 
proportions  in  which  they  are  found  at  high  ele- 
vations differ  but  little  from  those  at  sea-level. 

Thus  in  a  balloon  ascent  at  Kew,  the  percent- 
age of  oxygen  present  at  a  height  of  18,630  feet 
was  found  to  be  20.88,  while  it  was  20.92  at  the 
surface.  Here  it  varies  chiefly  according  to  the 
lack  of  ventilation  and  the  number  of  people  who 
inhabit  confined  spaces.  In  the  pit  of  a  theatre 
the  percentage  is  20.7,  in  a  law  court  20.6,  and  in 
the  gallery  of  a  theatre  about  20.5. 

So  far  as  its  chemical  properties  are  con- 
cerned, therefore,  the  atmosphere  at  great  heights 
is  just  as  suitable  for  man  as  it  is  at  sea-level. 
The  only  practical  drawbacks  arise  from  its 
greater  rarity  and  cold,  as  we  ascend  from  the 
surface.  /  v 

The  Nitrogen,  which  forms  three  fifths  of  the 
atmosphere,  represents  the  inert,  negative  ele- 
ment which,  though  not  actively  hostile  to  life, 
by  diluting  the  oxygen,  lessens  the  activity  and 
rapidity  of  the  energy  developed  by  the  latter's 
combustion,  and  thus  tends  to  prolong  life,  which 


20  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

would  be  used  up  too  rapidly  in  pure  oxygen.  It 
would  not  be  easy,  in  fact,  to  find  any  other  dilu- 
ent of  oxygen  which  could  take  the  place  of 
nitrogen  without  producing  poisonous  effects  like 
those  of  carbonic  acid. 

Regarded  from  a  physical  point  of  view,  nitro- 
gen, being  slightly  less  dense  than  oxygen  in  the 
proportion  of  97  to  no,  renders  the  air  a  better 
vehicle  for  sound,  support,  and  power  than  it 
would  be  otherwise. 

Nitrogen  is  also  absorbed  from  the  atmos- 
phere by  plants,  through  the  agency  of  those 
marvellous  little  bacilli  parasites,  the  Nitragin, 
which  have  recently  been  shewn  by  Prof.  Dobb6 
to  nourish  certain  plants  by  abstracting  the  nitro- 
gen from  the  air  and  passing  it  into  the  substance 
of  the  plants.  Each  plant,  moreover,  appears  to 
be  fed  by  its  own  special  bacillus,  but  starved  by 
that  of  any  other  plant. 

The  carbonic  acid  only  forms  a  very  small 
percentage  of  the  air,  but  nevertheless  plays  an 
important  part  in  the  operations  of  nature. 

Animals  consume  oxygen  and  exhale  carbonic 
acid  as  a  product  of  their  respiration.  Plants,  on 
the  other  hand,  under  the  action  of  light  on  their 
green  cells  decompose  the  carbonic  acid,  absorb 
the  carbon,  and  liberate  the  oxygen.  By  these 
means  the  balance  between  supply  and  consump- 
tion is  about  maintained. 

In  former  periods  of  the  earth's  history  the 
amount  of  carbonic  acid  in  the  atmosphere  was 
probably  much  greater  than  at  present.  Espe- 
cially during  the  carboniferous  epoch  of  geology, 
when  owing  to  special  climatic  conditions  enor- 
mous quantities  of  trees  and  ferns  grew  which 
abstracted  the  carbon  from  the  then  existing  at- 


NATURE  AND  COMPOSITION  OF  ATMOSPHERE.      21 

mosphere,  and  by  burying  it  for  centuries  in  the 
solid  form  of  coal  all  over  the  world  materially  re- 
duced the  subsequent  proportion  of  carbonic  acid 
from  what  had  previously  existed.  Though  .03 
per  cent.,  the  amount  existing  at  present  seems  a 
small  quantity,  it  is  yet  as  we  know,  enough  to 
supply  all  the  vegetable  world  with  its  solid 
carbon. 

Huxley  once  calculated  the  amount  of  this  gas 
which  is  contained  in  a  section  of  the  atmosphere 
resting  on  a  square  mile  to  be  as  much  as  13,800 
tons,  while  the  amount  of  solid  carbon  which  could 
be  extracted  from  such  a  quantity  of  the  gas  would 
be  about  3700  tons,  enough  to  supply  a  small  for- 
est of  trees  weighing  7400  tons. 

Ozone,  of  which  traces  exist  in  the  atmosphere, 
is  a  peculiar  form  of  oxygen,  a  molecule  of  which 
is  composed  of  two  atoms  linked  together,  and  a 
third  which,  on  the  principle  of  two  is  company 
and  three  is  none,  is  inclined  to  walk  off  whenever 
it  meets  with  a  suitable  companion.  Fortunately 
for  man  the  tastes  of  this  third  atom  are  distinctly 
low,  since  it  has  a  partiality  for  sewers  and  places 
where  matter  is  decomposing  and  which  by  its 
active  oxidising  power  it  renders  neutral  and 
harmless.  Since  towns  usually  contain  more  of 
such  deleterious  conditions  than  the  country, 
more  ozone  is  found  on  their  windward  than  on 
their  leeward  sides. 

Ozone  prevails  most  in  the  spring  months  and 
least  in  the  autumn,  and  while  it  probably  acts 
beneficially  as  a  rule,  by  its  active  oxidation  of 
poisonous  gases,  its  excess  is  associated  with  the 
prevalence  of  certain  forms  of  catarrhal  disease. 

Traces  of  ammonia  occur  which  help  to  supply 
nitrogen  to  the  soil  and  plants  when  washed  down 


22  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

by  rain.  Every  year  about  30  Ibs.  of  ammonia  arc 
carried  down  to  each  acre  of  ground.  The  above 
constituents  are  blended  together  like  different 
brands'  of  spirit,  but  are  free  to  enter  into  com- 
bination with  other  substances.  This  freedom  of 
contract  is  implied  in  the  term  mechanical  union, 
which  is  employed  to  distinguish  the  mixture  of 
oxygen  and  nitrogen  forming  atmospheric  air 
from  that  of  the  chemical  union  between  oxygen 
and  hydrogen  in  the  compound  water. 

The  vapour  of  water  which  as  an  invisible  gas 
is  generally  more  or  less  associated  with  dry  air 
may  be  looked  upon  as  a  separate  atmosphere  of 
gaseous  water.  The  fact,  however,  that  it  is  im- 
possible to  distinguish  it  from  dry  air  by  sight  or 
smell,  and  that  until  it  condenses  out  of  the  latter 
as  rain  or  cloud  it  virtually  forms  one  of  its  com- 
ponents, makes  it  desirable  for  us  to  regard  it  in 
this  light,  if  we  are  careful  to  remember  that  its 
quantity  (generally  about  i  per  cent,  by  weight) 
is  ever  varying,  and  that  the  volume  of  dry  air  it 
displaces  and  occupies  itself,  depends  on  the  tem- 
perature as  well-  as  the  mass  of  it  present.  When 
it  occurs  as  an  invisible  gas  it  is  |ths  as  dense  as 
dry  air  at  the  same  temperature  and  pressure. 
The  peculiarity  of  the  position  of  aqueous  vapour 
is,  that  if  it  existed  alone  on  the  earth,  there  would 
be  only  one  temperature  at  which  it  would  change 
from  a  gas  into  a  liquid,  and  therefore  only  one 
level  at  which  cloud  would  form  and  whence  rain 
would  descend  altering  with  the  time  of  day  and 
season. 

Since,  however,  it  exists  in  combination  with 
air,  it  spreads  upwards  until  it  arrives  at  the  par- 
ticular temperature  at  which  the  air  fails  to  sup- 
port it  in  solution,  when  a  layer  of  cloud  forms 


NATURE  AND  COMPOSITION  OF  ATMOSPHERE.      23 

and  perhaps  rain  falls.  After  this  an  interval  oc- 
curs in  which  the  vapour  is  at  first  in  defect,  but 
as  we  ascend,  its  relative  amount  to  that  which  is 
capable  of  being  sustained  increases  until  another 
level  and  temperature  is  reached  at  which  con- 
densation takes  place,  and  a  second  stratum  of 
cloud  is  formed  and  so  on.  Ultimately  a  point  is 
reached  at  which  the  vapour-sphere  nearly  van- 
ishes, but  this  must  be  very  high,  for  although  it 
is  found  that  at  a  height  of  23,000  feet  in  the 
Himalaya  the  amount  of  vapour  in  the  air  is  only 
one-tenth  of  that  which  exists  at  sea-level,  while 
at  46,000  feet  it  would  only  be  one  hundredth, 
cirrus  clouds  have  occasionally  been  seen  above 
the  latter  level. 

Dust  is  another  constituent  which  plays  an 
important  role.  Mr.  John  Aitken  of  Glasgow  has 
made  this  question  the  subject  of  special  investi- 
gation, and  has  found  that  the  atmosphere,  espe- 
cially in  its  lower  parts  over  land,  contains  thou- 
sands of  particles  of  the  finest  dust.  Over  the  sea 
and  in  its  loftier  regions  these  particles  are  much 
less  numerous.  He  has  also  found  that  the  presence 
of  this  dust  is  necessary  to  the  formation  of  rain. 

A  recent  series  of  observations  by  Mr.  E.  D. 
Fridlander,  taken  with  Aitken's  pocket  dust  coun- 
ter in  various  parts  of  the  world,  embracing  the 
Atlantic  and  Pacific  Oceans,  New  Zealand,  Cali- 
fornia, the  Indian  Ocean,  and  Switzerland,  shewed 
that  these  tiny  dust  particles  are  found  in  the 
lower  atmospheric  strata  right  out  in  the  middle 
of  the  Pacific  Ocean  as  well  as  on  land,  and  espe- 
cially in  towns.  They  are,  however,  less  numerous 
at  sea,  especially  in  the  Pacific  and  Indian  Oceans. 
Thus  comparing  all  three- oceans  we  have  at  sea- 
level. 


24  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

Number  of 
dust  particles  per 
cubic  centimetre.* 

Atlantic  Ocean  .  .  .  2053 

Pacific  .  .  .  613 

Indian         "  .  .  .  512 

As  low  a  value  as  210  was  found  in  the  Indian 
Ocean  after  rain.  On  the  other  hand,  over  land 
areas  the  number  frequently  rises  to  3000  or 
4000  per  cc.  In  large  cities  such  as  Edinburgh, 
Paris,  and  London,  where  the  products  of  animal 
and  fuel  combustion  enter  the  atmosphere  in 
large  quantities,  the  lower  atmosphere  is  so  pol- 
luted that  in  some  cases  as  much  as  150,000  dust 
particles  in  a  single  cubic  centimetre  have  been 
counted. 

As  we  rise  above  the  surface  the  number  of 
dust  particles  is  found  to  diminish  pretty  regu- 
larly with  the  ascent.  From  observations  on  the 
Bieshorn,  Fridlander  found  the  number  gradually 
diminish  in  the  following  ratio. 

Height  above  Number  of 

sea-level.  particles  per  cc. 

6,700  feet  950 


8,20O 
8,400 
10,665 
II.OOO 
13,200 
13,600 


480 
513 
406 
257 
2I9 
157 


The  general  rule  for  the  diminution  in  the 
number  of  dust  particles  may  be  simply  expressed 
thus:  For  every  rise  of  3000  feet  the  amount  is 
$ths  of  what  it  was  at  the  lower  level.  The  bear- 
ing of  this  fact  on  the  question  of  the  beneficial 
influence  of  high  mountain  resorts  on  pulmonary 
and  other  diseases  is  obvious. 


*  About  15  cubic  centimetres  are  equal  to  i  cubic  inch. 


PRESSURE   AND   WEIGHT   OF  ATMOSPHERE.     25 

These  same  minute  dust  particles,  by  their 
scattering  action  on  the  small  waves  of  light  at 
the  violet  end  of  the  spectrum,  have  been  shewn 
by  Lord  Rayleigh  to  be  the  cause  of  blue  sky, 
while  its  gradual  deepening  into  black  as  we 
ascend  is  readily  seen  to  be  the  result  of  their 
gradual  diminution  in  number. 


CHAPTER   III. 

THE    PRESSURE    AND    WEIGHT    OF    THE 
ATMOSPHERE. 

ONE  of  the  first  facts  which  is  brought  to  our 
notice  in  these  days  when  those  physical  laws, 
which  the  ancient  philosophers  discovered  to- 
wards the  end  of  their  lives,  are  taught  us  from 
childhood,  is  that  the  air  has  weight  and  exerts 
pressure.  The  story  of  the  discovery  of  the  bar- 
ometer or  weight  measurer  is  a  romantic  chapter 
in  the  history  of  science. 

About  1643,  some  Florentine  gardeners  found 
that  they  were  unable  to  pump  up  water  higher 
than  thirty-three  feet.  Up  to  that  time  it  was  an 
accepted  dogma  that "  Nature  abhorred  a  vacuum," 
and  this  apparent  lapse  on  the  part  of  Nature  was 
looked  upon  as  inexplicable.  When  Galileo  was 
informed  of  it,  soured  as  he  was  with  a  world 
which  had  rejected  some  of  his  greatest  discov- 
eries, he  cynically  remarked  that  Nature  evi- 
dently abhorred  a  vacuum  up  to  thirty-three  feet. 
His  pupil,  Torricelli,  however,  was  not  content 
with  this  perfunctory  explanation,  and  applying 


Z6     THE   STORY  OK  THE  EARTH'S   ATMOSPHERE. 

his  genius  to  the  question,  conjectured  that  the 
column  of  thirty-three  feet  of  water  exactly  bal- 
anced a  similar  column  of  air  stretching  to  the 
limits  of  the  atmosphere.  Remembering  that 
mercury  was  about  thirteen  times  as  heavy  as 
water,  he  inferred  that  if  this  were  true,  a  mer- 
cury pump  would  only  raise  mercury  to  a  height 
of  about  30  inches.  He  thereupon  filled  a  long 
glass  tube  with  mercury,  and  having  stopped  up 
one  end,  placed  his  thumb  over  the  open  end  and 
inverted  it  over  a  basin  of  the  liquid  metal.  The 
result  proved  his  anticipations  to  have  been  well 
founded,  since  the  mercury  fell  in  the  tube  until 
it  exactly  reached  this  height  of  30  inches,  leav- 
ing what  is  known  as  the  Torricellian  vacuum  in 
the  upper  part  of  the  tube. 

This  is  substantially  the  mercurial  barometer 
by  which  to-day  we  measure  what  we  term  atmos- 
pheric pressure. 

The  reason  the  itvm  pressure  is  employed  and 
not  weight  is  because  air,  in  common  with  all 
fluids,  not  merely  presses  downwards,  but  equally 
in  all  other  directions. 

This  is  readily  shewn  by  the  familiar  experi- 
ment of  placing  a  bit  of  paper  over  the  mouth  of 
a  bottle  full  of  water,  and  inverting  it,  when  the 
water  will  be  retained  by  the  upward  pressure  of 
the  air  on  the  surface  of  the  paper. 

When  we  want  to  measure  the  weight  of  air, 
we  must  remember  that,  since  air  is  elastic,  it  is 
more  compressed,  and  therefore  weighs  heavier 
near  the  surface  than  up  above. 

At  sea-level,  where  the  barometer  frequently 
registers  a  height  of  30  inches,  we  shall  find  that 
at  32°  Fahr.  the  column  of  mercury  30  inches 
high  resting  on  one  square  inch  weighs  14.7  Ibs. 


PRESSURE  AND  WEIGHT  OF  ATMOSPHERE.     27 

It  is  easy  from  this,  knowing  that  mercury  is  13.6 
times  as  dense  as  water,  and  air  only  xoV^ths  as 
dense,  to  measure  the  weight  of  a  cubic  foot  of 
pure  dry  air,  which  under  these  conditions  will  be 
about  565  grains  (troy).  On  the  top  of  a  moun- 
tain 18,000  feet  high  it  would  only  weigh  half  as 
much.  The  weight  of  a  cubic  foot  of  water  va- 
pour under  the  same  conditions  would  be  only 
352  grains.  From  this  it  will  be  understood  that, 
when  vapour  is  mixed  with  dry  air,  the  resulting 
compound  is  lighter — that  is,  damp  air  is  lighter 
than  dry  air. 

The  weight  of  the  atmosphere  on  the  earth 
cannot  be  ignored. 

A  flood  of  water  33  feet  high  over  the  globe 
would  represent  the  same  weight,  and  would  evi- 
dently exercise  a  very  considerable  pressure  on 
the  surface.  Westminster  Hall  alone  contains  75 
tons  of  air,  while  the  entire  weight  of  air  resting 
on  the  earth  has  been  estimated  by  Sir  John 
Herschel  to  amount  to  uf  trillions  of  pounds. 
Sudden  alterations  of  this  pressure,  which  are  in- 
dicated by  the  rise  and  fall  of  the  barometer,  un- 
doubtedly affect  some  persons  of  a  sensitive  tem- 
perament, while  the  steady  fall  of  pressure  which 
occurs  when  we  ascend  a  mountain  or  rise  in  a 
balloon  occasions  what  is  termed  mal  de  montagne 
in  both  men  and  animals. 

On  the  other  hand,  the  excessive  pressure  ex- 
perienced in  diving-bells  or  caissons,  or  in  the 
digging  of  tunnels,  where  the  men  work  under  a 
pressure  of  two  or  more  atmospheres,  is  found  to 
bring  on  a  species  of  paralysis. 

To  give  a  general  idea  of  the  decrease  of  pres- 
sure with  the  height  when  the  barometer  marks 
30  inches  at  sea-level,  we  find  the  following  rela- 


28     THE   STORY   OF   THE   EARTH'S   ATMOSPHERE. 

tive  scale  for  air  of  an  average  temperature  and 
dampness. 

Pressure.  Altitude. 

30  inches         .....  o 


910 

1,850 
2,820 


3,820 

4,850 

5,910 

7,oio 

8,150 

9,330 

10,550 

18      "             13,170 

16      "             16,000 


At  18,000  feet  the  pressure  is  about  half  that 
at  sea-level. 

It  will  be  observed  that  at  the  lower  eleva- 
tions the  height  in  feet  corresponding  to  one 
inch  in  the  barometer  is  less  than  at  the  higher. 
The  atmosphere  is  in  fact  more  tightly  packed 
near  the  earth,  so  that  while  i  inch  of  mercury 
represents  the  weight  of  the  first  900  feet  of 
ascent,  i  inch  at  16,000  feet  represents  the  weight 
of  about  1500  feet,  and  the  proportion  increases 
at  greater  heights. 

Were  the  scale  i  inch  of  mercury  to  910  feet 
of  atmospheric  air  preserved  all  the  way  up,  we 
should  reach  the  limit  of  the  atmosphere  at  about 
26,220  feet,  or  5  miles,  which  is  the  height  of  what 
is  termed  a  homogeneous  atmosphere. 

Comparing  the  atmosphere  with  the  ocean,  we 
find  that  the  volume  of  the  former,  assuming  it  to 
reach  to  a  height  of  100  miles,  is  as  65  to  i,  while 
its  mass  bears  to  that  of  the  latter  the  ratio  of 
only  i  to  300. 

The   pressure  at   the   average  depth   of   the 


PRESSURE    A^D   WEIGHT   OF   ATMOSPHERE.     29 

ocean — viz.,  two  miles,  is  as  much  as  320  atmos- 
pheres. 

The  barometric  pressure  undergoes  changes, 
some  of  which  are  irregular,  and  due  to  the  pas- 
sage of  what  are  termed  cyclones  and  anticy- 
clones, in  which  the  air  is  moving  round  moving 
centres,  while  others,  such  as  those  which  complete 
their  period  in  a  year,  are  connected  with  seasonal 
transfers  of  air  between  sea  and  land  and  from 
hemisphere  to  hemisphere.  Others,  again,  which 
run  through  their  course  in  a  day,  are  connected 
with  the  daily  heating  and  cooling  of  the  air  by 
the  sun,  while  certain  short  and  nearly  regular 
instantaneous  changes  over  large  areas,  such  as 
the  five-day  pressure  oscillations  recently  noticed 
by  Eliot  in  India,  are  still  mysteries  that  require 
explanation.  The  seasonal  changes  and  the  gen- 
eral distribution  of  pressure  will  be  alluded  to  in 
future  chapters,  where  they  are  considered  with 
reference  to  dependent  phenomena. 

The  diurnal  variation  of  barometric  pressure 
which  is  dependent  on  the  daily  rise  and  fall  of 
sun-heat  is  largest,  as  we  should  expect,  in  the 
tropics,  amounting  to  a  range  of  as  much  as 
twelve  hundredths  of  an  inch  at  Calcutta,  and 
diminishing  thence  as  we  travel  polewards,  until 
at  Greenwich  it  is  only  about  .02  inch,  or  one- 
sixth  of  its  tropical  value.  Nearer  the  poles  it 
vanishes  altogether.  Between  the  tropics,  the 
irregular  changes  of  pressure  introduced  by  the 
passage  of  storms  are  so  small  and  infrequent 
that  the  diurnal  variation  is  noticeable  above  all 
other  changes,  and  is  so  regular  that  the  late  Mr. 
Broun,  of  Trevandrum  Observatory  in  India,  used 
to  declare  he  could  tell  the  time  of  day  by  simply 
noting  the  height  of  the  barometer.  The  rise  and 


30  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

fall  of  the  mercury  column  is  a  double  one,  reach- 
ing its  greatest  height  at  10  A.  M.  and  10  p.  M.,  and 
its  least  height  at  4  A.  M.  and  4  P.  M.  The  causes 
are  not  yet  thoroughly  worked  out,  since,  al- 
though it  undoubtedly  depends  on  the  action  of 
the  sun,  the  total  effect  is  made  up  of  a  combina- 
tion of  direct  and  indirect  motions  of  the  air.  In 
temperate  regions  the  diurnal  change  of  pressure 
is  so  small  that  it  is  almost  lost  sight  of  in  those 
much  larger  pressure  changes  introduced  by  the 
passage  of  cyclones,  which  frequently  amount  to 
i  or  2  inches'  rise  and  fall  of  the  mercury. 

Barometric  charts  in  which  isobars,  or  lines  of 
equal  barometric  pressure,  are  drawn  over  the 
representation  of  different  parts  of  the  earth,  will 
be  referred  to  in  chap.  V.  These  charts  are  simi- 
lar to  those  employed  in  weather  bureaux  in  or- 
der to  forecast  the  probable  weather  for  the  ensu- 
ing twenty-four  hours. 

One  practical  use  of  the  barometer  is  to  deter- 
mine the  altitude  of  a  place  above  sea-level.  The 
science  of  measuring  heights  by  this  means  is 
termed  hypsometry  (from  the  Greek,  hypsos, 
height,  metron,  measure).  We  have  already  seen 
that  the  pressure  descends  in  a  certain  proportion 
as  we  ascend  in  the  atmosphere,  and  formulae 
have  been  determined  by  which  the  height  may  be 
calculated  under  certain  conditions  of  tempera- 
ture, humidity,  etc.  For  rough  and  ready  pur- 
poses, however,  the  following  rule  gives  a  very 
fair  approximation  : — 

"  The  difference  of  level  in  feet  between  two  alti- 
tudes is  equal  to  the  difference  of  the  barometric  pres- 
sures observed  at  each  in  inches  divided  by  their  sum 
and  multiplied  by  the  number  55,76*,  when  the  ai>er- 
age  of  the  temperatures  at  the  two  places  is  60°  f." 


THE  TEMPERATURE  OF  THE  ATMOSPHERE.  31 

When  the  average  temperature  of  the  two  sta- 
tions is  above  60°  the  multiplier  must  be  increased 
by  117  for  every  degree  the  average  is  above  this 
temperature,  and  decreased  in  like  manner  for 
every  degree  it  is  below  60°.  Thus,  if  the  values 
at  the  lower  station  are  30.15  inches  pressure  and 
65°  temperature,  and  those  at  the  upper  station 
are  28.67  inches  and  59°,  a  little  household  arith- 
metic will  shew  that  the  difference  of  their  heights 
is  1409  feet. 


CHAPTER   IV. 

THE    TEMPERATURE    OF    THE    ATMOSPHERE. 

THE  temperature  of  the  atmosphere,  whether 
we  are  aware  of  it  or  not,  is  a  condition  in  which 
we  are  more  directly  interested  than  any  other. 
The  most  common  form  of  salutation  in  the  street 
involves  a  dictum  or  a  query  as  to  "how  cold  it 
is  to-day,"  "much  warmer  than  yesterday,"  "I  do 
hope  we  are  going  to  have  some  really  warm 
weather  now,"  or  "some  skating,"  as  the  case 
may  be.  In  all  this  the  temperature  of  the  air  is 
concerned,  since  it  is  the  medium  in  contact  with 
us,  and  from  which,  chiefly  by  conduction,  we  de- 
rive our  sensation  of  heat  or  cold.  When  we  talk 
of  temperature  we  must  take  care  to  know  what 
we  mean  by  the  term.  Heat,  as  we  know,  is  a 
"  mode  of  motion,"  as  Tyndall  used  to  call  it,  a 
vibration  of  the  small  molecules  of  a  body,  and 
directly  this  mode  of  motion  is  communicated  to 
it,  by  what  is  termed  radiation,  it  tends  to  return 
the  compliment  to  other  bodies  in  its  neighbour- 
hood, and  set  all  their  molecules  in  a  similar  state 
3 


32  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

of  oscillation.  The  process,  however,  is  an  ex- 
change all  round,  and  the  temperature  of  any  body 
measures  the  rate  at  which  it  loses  heat  to  or  gains 
heat  from  surrounding  bodies.  This  rate  depends 
upon  its  capacity  for  heat,  and  its  power  of  ab- 
sorbing and  radiating  heat  rays,  all  of  which  vary 
in  different  bodies. 

In  the  case  of  the  atmosphere,  the  radiating 
power  exceeds  the  absorbing  power  for  rays  com- 
ing from  the  sun,  but  is  considerably  less  for  the 
heat  radiated  back  again  from  the  earth.  So  that, 
on  the  whole,  the  absorption  power  of  the  lower 
air  for  all  kinds  of  rays  is  about  2^  as  great  as 
its  radiation  power. 

It  is  this  property  of  the  atmosphere  which  al- 
lows us  to  keep  decently  warm.  Otherwise,  were 
we  bereft  of  this  valuable  covering  or  envelope 
we  should  shiver  in  a  temperature  of  138  degrees 
below  zero  Fahrenheit,  which  is  probably  the 
mean  temperature  of  the  moon's  surface.  The 
only  advantage  that  could  be  claimed  for  such  a 
temperature  is,  that  it  would  be  332  degrees  higher 
than  what  would  probably  ensue  in  the  event  of 
the  sun  becoming  cold. 

The  temperature  of  the  atmosphere  is  derived 
chiefly  from  the  solar  radiation  which  is  arrested 
by  the  earth,  and  partly  reflected,  partly  radiated 
back  through  the  atmosphere  towards  space. 
Temperature  is  a  result  of  radiation. 

Consequently  before  we  speak  of  the  tempera- 
ture it  is  necessary  to  see  how  radiation  affects 
the  atmosphere,  since  the  conditions  which  regu- 
late radiation,  affect  the  temperature  of  the  at- 
mosphere in  a  somewhat  similar  manner. 

When  the  sun's  radiations  have  reached  the 
earth's  surface  from  which  the  lowest  stratum  of 


THE  TEMPERATURE  OF  THE  ATMOSPHERE.  33 

the  atmosphere  chiefly  derives  its  temperature, 
their  heating  effect  on  a  given  area  is  modified  by 
two  circumstances,  (i)  their  angle  of  incidence  or 
the  angle  the  direction  of  the  sun  makes  with  the 
horizon,  and  (2)  the  thickness  of  atmosphere  they 
have  traversed. 

When  a  certain  width  of  the  sun's  rays  is  con- 
sidered it  will  be  found  to  cover  a  smaller  area  in 
proportion  as  they  fall  more  vertically  or  less  in- 
clined. Thus  in  the  accompanying  diagram  the 

Sun  vertical  (    at  noon 
Equinox  on  Equator). 


same  width  of  rays  is  concentrated  upon  A  B  in 
the  one  case,  and  spread  over  A  C  in  the  other, 
consequently  the  heat  received  by  the  earth  is 
greatest  when  the  sun  is  highest  above  the  hori- 
zon, and  shines  most  directly  upon  the  ground. 
During  a  single  day  the  heat  received  on  the 
ground  is  greater  at  noon  than  at  any  other  hour 
(about  four  times  as  great  as  at  10  A.  M.  or  2  p.  M.). 
It  is  also  greater  in  the  summer  when  the  sun  is 
permanently  at  a  higher  angle  all  through  the  day 
after  it  has  risen,  than  it  is  in  the  winter.  These 
both  operate  together  at  any  place  on  the  earth. 
When  we  change  our  latitude  we  can,  by  travel- 


34     THE   STORY  OF  THE   EARTH'S  ATMOSPHERE. 

ling  towards  or  from  the  equator  at  the  rate  of 
about  18  miles  per  day,  obviate  the  seasonal 
change  in  the  angle  of  the  sun  above  the  horizon 
and  secure  the  same  general  amount  of  sun  radia- 
tion. We  should  not,  however,  be  able  to  secure 
the  same  average  temperature  since  the  direct  ef- 
fects of  the  radiation  on  temperature  are  modified 
by  what  goes  on  over  entire  hemispheres.  More- 
over the  effect  of  changing  our  latitude  introduces 
another  consideration  which  has  a  potent  influence 
upon  the  amount  of  heat  falling  in  the  24  hours 
— viz.,  the  time  during  which  the  sun  remains 
above  the  horizon.  This  time  increases  as  we 
travel  polewards  in  the  hemisphere  which  is  en- 
joying summer.  There  are  thus  two  influences 
which  work  in  opposite  directions,  one,  the  gen- 
eral angle  of  the  sun  above  the  horizon,  which 
diminishes  as  we  leave  the  equator,  and  the  other, 
the  length  of  the  day,  which  increases  under  the 
same  conditions.  The  conjoint  effect  must  there- 
fore generally  reach  its  maximum  value  at  some 
intermediate  latitude. 

As  a  matter  of  fact,  this  important  problem 
has  been  worked  out  by  several  physicists,  in- 
cluding Lambert,  Poisson,  and  Meech.  The  last- 
named  finds  that  on  the  average  of  the  year,  as 
we  should  expect,  more  heat  falls  on  the  equator 
than  elsewhere.  If  we  take  the  six  months  of  the 
northern  summer,  more  heat  falls  on  latitude  25 
degrees  north  (the  latitude  of  northern  India)  than 
on  the  equator.  If  again  we  take  the  three 
months  nearest  midsummer,  i.e.  from  May  7th  to 
August  yth,  the  zone  of  greatest  heat  reception 
lies  in  41°  N.,  while  from  May  3ist  to  July  i6th, 
more  heat  falls  on  the  North  Pole  than  on  any 
other  part  of  the  earth.  The  temperature  of  the 


THE  TEMPERATURE  OF  THE  ATMOSPHERE.  35 

Pole  does  not  of  course  at  once  respond  to  this 
heating,  since  the  average  temperature  effect  lags 
about  one  month  behind  the  solar  radiation,  and 
near  the  Pole  the  heat  is  mainly  employed  in  melt- 
ing the  Arctic  ice  floes,  and  in  raising  the  temp- 
erature of  the  water.  At  the  same  time  this 
beneficial  arrangement  obviously  prevents  the 
temperature  there  from  becoming  as  low  as  it 
otherwise  would. 

In  addition  to  this,  the  amount  of  heat  which 
is  transmitted  through  the  atmosphere  so  as  to 
reach  the  surface  at  all,  varies  with  the  angle  of 


FIG.  5. 

the  sun  for  a  different  cause — viz.,  the  different 
thickness  of  the  atmosphere  traversed  in  each  case. 

This  is  plain  from  the  adjoining  figure  in  which 
as  the  sun's  rays  fall  vertically  or  inclined,  we 
have  the  thicknesses  A.P.,  B.P.,  and  C.P. 

This  last  circumstance  exaggerates  the  differ- 
ence caused  by  the  hourly  and  seasonal  changes 
in  the  angle  of  the  sun,  especially  as  it  approaches 
the  horizon. 


36  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

Direct  observations  of  the  sun-heat  by  means 
of  an  instrument  termed  an  actinometer,  which 
has  been  employed  with  great  success  by  Prof.  S. 
P.  Langley  at  Washington,  have  shown  that  of 
the  heat  which  falls  vertically  on  the  upper  sur- 
face of  the  atmosphere,  25  per  cent  is  absorbed 
(Langley  says  40  per  cent,  but  this  seems  doubt- 
ful from  other  considerations)  before  it  penetrates 
to  the  earth.  When  the  rays  are  inclined,  instead 
of  75  per  cent  being  transmitted,  only  64  per  cent 
arrives  at  an  angle  of  50  degrees,  and  only  16  per 
cent  at  an  angle  of  10  degrees.  The  light  varies 
in  the  same  way.  At  sunrise  and  sunset  the  sun 
has  only  -j-g^th  part  of  the  brilliancy  it  possesses 
when  vertical  overhead. 

When  we  come  to  consider  the  actual  quantity 
of  heat  that  is  received  from  the  sun,  we  shall  see 
how  utterly  it  transcends  all  our  means  for  deriv- 
ing warmth  from  (so-called)  artificial  sources. 
The  intensity  of  solar  heat  may  be  measured  by 
the  temperature  to  which  it  would  raise  a  certain 
quantity  of  water.  If  we  suppose  the  rays  which 
fall  vertically  on  an  area  one  square  foot  at  the 
outside  of  the  atmosphere,  before  any  absorption 
has  taken  place  to  be  applied  to  warming  up  10 
Ibs.  o^f  water,  they  would  raise  it  i  degree  on  a 
Fahrenheit  thermometer  in  i  minute. 

By  the  time  these  rays  have  reached  the  earth, 
as  we  have  seen,  about  £th  of  the  original  radiation 
has  been  absorbed  or  scattered  by  the  atmosphere, 
and  therefore  only  about  7  Ibs.  of  water  could  be 
raised  i  degree  per  minute.  This  however  gives 
us  some  faint  idea  of  the  enormous  quantity  of 
heat  which  is  continually  falling  on  either  the 
earth  or  the  clouds.  If  we  take  the  heat  which 
falls  on  a  square  mile  of  the  earth's  surface  per 


THE  TEMPERATURE  OF  THE  ATMOSPHERE.  37 

minute,  we  shall  find  that  it  would  be  enough  to 
raise  560  tons  of  water  from  the  freezing  to  the 
boiling  point. 

In  a  year,  assuming  that  the  sun's  heat  con- 
tinually penetrated  to  the  ground,  this  heat  would 
suffice  to  melt  a  layer  of  ice  about  178  feet  thick 
over  the  whole  earth,  or  not  much  below  the 
monument  in  London. 

The  general  effect  has  been  popularly  put  by 
one  writer  in  the  following  graphic  manner,  in 
which  the  different  amount  of  heat  received  when 
the  sun  is  inclined  at  different  angles  is  properly 
considered. 

"  Suppose  the  earth  one  vast  stable  covered 
with  horses,  and  suppose  that  as  the  sun's  angle 
varied  according  to  season  and  latitude,  the  horses 
arranged  themselves  so  that  no  horse's  shadow 
fell  upon  or  underneath  his  neighbour;  then  the 
solar  heat  falling  upon  the  earth  converted  into 
horse  power,  would  be  always  represented  by  all 
these  horses  working  continuously  at  their  utmost 
strength." 

Some  of  this  heat  energy  is,  no  doubt,  con- 
verted into  mechanical  energy  in  the  winds,  rivers, 
and  rainfall,  but  a  vast  proportion  of  it  is  wasted 
so  far  as  man  is  concerned,  and  it  is  plain,  as  both 
Lord  Kelvin  and  Edison  have  recently  pointed 
out,  that  we  have  still  an  immense  source  of  power 
comparatively  untouched,  which  can  be  dratfn 
upon  when  our  coal  supply  shows  symptoms  of 
giving  out. 

One  effect  has  not  been  alluded  to — viz.,  the 
change  in  the  distance  between  the  earth  and  the 
sun,  which  are  nearer  to  one  another  in  December 
than  in  July.  Theoretically  the  effect  would  in 
any  case  be  small.  Practically  it  is  counteracted 


38  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

by  the  large  mass  of  water  in  the  southern  hemi- 
sphere, which  responds  more  slowly  to  an  increase 
of  heat  than  the  northern  land,  so  that  on  latitude 
20  degrees  S.,  where  it  reaches  its  greatest  effect, 
it  only  adds  ^th  to  what  would  occur  if  the  dis- 
tance were  invariable. 

Since  the  temperature  of  the  atmosphere  re- 
sults from  the  accumulation  of  altered  solar  rays, 
in  surrounding  objects  which  radiate  them  to  one 
another,  instead  of  passing  them  back  at  once 
into  space,  the  temperature  epochs  will  always 
follow  those  of  direct  radiation.  Thus  the  highest 
temperature  of  the  day  does  not  occur  at  noon, 
but  an  hour  or  two  afterwards.  Similarly  the 
highest  temperature  of  the  year  occurs  on  an 
average  a  month  after  midsummer  day,  and  a  like 
retardation  occurs  for  the  lowest  temperatures. 
At  the  Pole,  where  one  long  day  and  night  occurs 
in  the  year,  the  coldest  month  is  delayed  to  Feb- 
ruary or  March,  in  the  northern  hemisphere. 
When  the  sun's  rays  fall  upon  water,  or  where  the 
locality  is  naturally  moist,  the  heat  is  conducted 
through  the  top  layer,  and  in  any  case  takes  longer 
to  raise  its  temperature.  Where,  as  always  occurs, 
part  of  the  water  is  evaporated,  nearly  1000  times 
as  much  heat  is  needed  to  convert  it  into  vapour 
as  will  raise  its  temperature  i  degree  Fahr.  Con- 
sequently, not  only  does  the  temperature  of  the 
air  over  oceans  rise  and  fall  less  daily  and  sea- 
sonally than  that  over  the  continents,  but  the 
highest  temperature  of  the  year  in  maritime 
regions  lags  about  42  days  behind  midsummer 
day,  while  in  the  centre  of  the  large  continents, 
the  lag  is  reduced  to  25  days. 

This  slowness  to  rise  and  fall  in  temperature 
on  the  part  of  large  masses  of  water,  accounts  for 


THE  TEMPERATURE  OF  THE  ATMOSPHERE.  39 

the  equable  temperature  of  the  atmosphere  of 
islands  and  coasts,  compared  with  interiors  of 
continents,  and  exerts  an  important  influence  in 
determining  the  changes  in  the  general  wind  and 
weather  system  over  oceans  and  continents  in 
summer  and  winter. 

The  measurement  of  atmospheric  temperature 
dates  back  like  that  of  the  telescope  to  Galileo, 
who  in  1597  devised  the  first  liquid  thermometer. 

This  consisted  of  a  glass  bulb,  containing  air, 
terminating  below  in  a  long  glass  tube,  which 
dipped  into  a  vessel  containing  a  coloured  fluid. 
The  variation  of  the  volume  of  the  enclosed  air, 
caused  the  fluid' to  rise  and  fall  in  the  tube  to 
which  an  arbitrary  scale  was  attached.  Galileo 
further  invented  the  alcohol  thermometer  in  1611, 
which  was  adopted  generally  by  the  Florentines 
of  that  time. 

The  determination  of  the  zero  and  some  fixed 
point  above  it,  by  which  to  graduate  the  scale, 
appears  to  have  taken  years  to  evolve.  Newton 
suggested  a  scale  in  which  the  freezing  point  of 
water  was  o  degrees,  and  the  blood  of  a  healthy 
man  12  degrees,  and  subsequently  Fahrenheit,  to 
whose  scale  with  characteristic  conservatism  we 
still  adhere  in  this  country,  in  spite  of  the  uni- 
versal use  of  that  of  Celsius  on  the  continent  and 
in  physical  investigation,  in  1714  took  blood  heat 
and  that  of  a  freezing  mixture  of  ice  and  salt  as 
his  fixed  points.  Since  then  the  freezing  and  boil- 
ing points  of  water  have  been  taken  as  the  fixed 
points  on  the  thermometric  scale. 

Unlike  the  early  Florentine  thermometers,  the 
modern  alcohol  and  mercury  thermometers  con- 
sist of  a  bulb  and  tube,  partially  filled  with  the 
liquid,  above  which  is  a  tolerably  complete  vacu- 


40  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

um,  allowing  the  liquid  to  move  with  perfect 
freedom  up  and  down  the  tube. 

For  measuring  rapid  variations  in  the  temper- 
ature of  the  atmosphere,  it  is  necessary  to  have 
the  bulb  small,  since  where  the  bulb  is  large,  the 
effect  of  an  exposure  to  heat  is  considerably 
delayed.  Consequently,  for  determining  the  true 
shade  temperature  of  the  atmosphere  at  any 
moment  where  it  is  difficult  to  obtain  proper 
shade  conditions,  a  small  sling  thermometer  is 
by  far  the  most  accurate.  By  tying  any  ther- 
mometer to  a  string,  and  whirling  it  round  un- 
til the  reading  does  not  alter,  a  very  fair  notion 
of  the  true  temperature  of  the  air  can  be  ob- 
tained. 

For  standard  purposes  where  momentary 
changes  are  not  so  important,  thermometers  with 
large  bulbs  are  preferred,  since  by  this  plan  the 
variations  due  to  the  expansibility  of  the  glass 
bear  a  small  ratio  to  the  volume  of  mercury. 

We  have  now-a-days  advanced  so  rapidly  in 
our  methods  of  investigation,  that  instead  of  be- 
ing content  with  two  or  three  readings  a  day,  we 
require  to  know  the  continuous  changes  in  the 
temperature  of  the  atmosphere  in  places  where  it 
is  impossible  to  make  eye  observations. 

For  this  purpose  the  self-recording  thermo- 
graph is  employed,  and  by  the  use  of  such  an  in- 
strument the  temperature  of  the  atmosphere  can 
be  registered  on  the  top  of  mountain  peaks  only 
occasionally  accessible,  and  in  the  free  atmosphere 
by  the  elevating  power  secured  by  kites  and  cap- 
tive or  free  balloons. 

When  we  examine  the  observed  facts  as  they 
present  themselves,  we  find  in  the  first  place  a 
constant  diminution  of  the  temperature  of  the 


THE  TEMPERATURE  OF  THE  ATMOSPHERE.  41 

atmosphere  as  we  ascend  from  the  earth's  sur- 
face. This  decrease  of  temperature  with  ascent 
varies  somewhat  in  different  latitudes,  and  is  not 
the  same  in  the  free  atmosphere  as  on  mountain 
sides. 

From  Glaisher's  balloon  ascents  the  rate  be- 
gins quite  near  the  surface  at  about  7  degrees  in 
every  140  feet,  and  finally  diminishes  to  i  de- 
gree in  every  400  feet  at  10,000  feet,  the  entire 
diminution  of  temperature  from  sea-level  up  to 
10,000  feet  being  34  degrees,  or  i  degree  in  300 
feet.  If  therefore  we  take  the  temperature  of 
London  to  be  about  50  degrees  on  the  mean  of 
the  year,  a  temperature  of  freezing  point  or  in 
other  words  the  snow  line  would  be  reached  at  an 
elevation  of  about  4500  feet  or  a  little  above  Ben 
Nevis  in  the  free  atmosphere  (the  mean  tempera- 
ture at  the  top  of  Ben  Nevis  is  about  30^  degrees 
F.).  No  higher  mountains  exist,  consequently 
there  are  no  perpetual  snows  in  the  British 
islands. 

In  India  the  initial  rate  of  decrease  of  tem- 
perature is  very  much  more  rapid,  amounting  to 
i  degree  in  the  first  33  feet,  but  it  slackens  down 
to  about  i  degree  in  330  feet  at  15,000  feet.  On 
the  average  it  is  about  i  degree  in  270  feet  in  sta- 
tions away  from  the  Himalaya  where  the  moun- 
tain range  appears  to  reduce  the  rate. 

If  we  take  the  region  of  the  North  West 
Himalaya  we  shall  find  that  the  mean  tempera- 
ture of  London  would  be  reached  at  a  height  of 
9600  feet,  and  the  range  of  temperature  through- 
out the  year  would  not  differ  very  much  from 
that  of  England. 

Most  of  the  Himalayan  sanitaria  lie  between 
6000  and  7000  feet  where  the  temperature  is 


42     THE   STORY  OF  THE   EARTH'S  ATMOSPHERE. 

about  60  degrees,  and  possess  climates  similar 
to  those  of  the  Riviera  and  the  coast  from  Mar- 
seilles to  Genoa. 

When  therefore  we  wish  to  vary  our  climate 
as  far  as  temperature  is  concerned,  we  can  do  so 
without  changing  our  latitude  by  remembering 
that  the  temperature  cools  on  an  average  about  i 
degree  for  every  300  feet  we  ascend,  or  warms  at 
the  same  rate  as  we  descend  the  same  distance. 
Since  the  mean  temperature  at  the  north  pole  is 
about  o  degree  F.  and  at  the  equator  between  80 
and  90  degrees  F.,  we  can  similarly  alter  our  tem- 
perature i  degree  F.  by  travelling  north  or  south 
about  70  to  80  English  miles.  As  an  illustration 
of  a  combination  of  these  facts  we  can  imagine 
a  series  of  planes  rising  upwards  from  different 
points  of  the  earth  towards  the  equator  along 
which  the  temperature  would  range  on  either  side 
of  a  certain  average  throughout  the  year.  These 
would  rise  to  their  highest  level  over  the  equator, 
and  their  height  in  any  latitude  would  show  us  at 
what  elevation  we  should  experience  some  par- 
ticular temperature  all  the  year  round. 

The  vertical  scale  above  sea-level  is  of  course 
immensely  exaggerated. 

It  will  be  seen  that  at  an  elevation  of  27,000 
feet  over  the  equator,  the  temperature  is  about  o 
degree  F.,  and  that  the  snow  line  or  line  of  freez- 
ing point  cuts  the  surface  at  sea-level  about  lati- 
tude 69°  North. 

In  the  Himalaya  this  line  throughout  the  year 
is  about  15,400  feet  above  the  sea,  or  about  17,850 
feet  in  the  summer  months.  Even  this  great  alti- 
tude would  still  leave  about  11,000  feet  of  the 
higher  summits  mantled  with  perpetual  snow  dur- 
ing the  summer. 


THE  TEMPERATURE  OF  THE  ATMOSPHERE.  43 

There  is  perhaps  no  point  about  which  so 
much  perplexity  is  generally  felt  and  expressed 
as  the  reason  for  this  decrease  of  temperature  as 
we  ascend.  It  is  often  popularly  expressed  as  be- 


Centre  of 
Earffi 


FIG.  6. 


ing  due  to  the  greater  rarity  of  the  air  above,  but 
this  simply  leaves  the  matter  as  obscure  as  before. 
Like  most  other  facts  regarding  the  atmosphere, 
it  results  from  the  operation  of  a  definite  physical 
law. 

It  is  well  known  that  the  rapidity  of  the  cool- 
ing of  a  body  depends  on  the  perfection  of  its  en- 
closure, whether  solid  or  gaseous.  At  the  earth's 
surface  the  enclosure  is  nearly  perfect,  but  as  we 
ascend,  the  upper  side  of  the  enclosure  weakens 
owing  to  the  thinning  of  the  air,  until  at  the  top 
of  the  atmosphere  the  enclosure  is  only  half  what 


44  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

it  was  at  the  earth's  surface.  The  heat  radiated 
from  the  earth  is  moreover  intercepted  to  a  large 
extent  at  the  higher  levels  by  the  intervening 
lower  air.  Consequently  on  both  accounts  the 
temperature  of  the  air  remains  cooler  in  propor- 
tion to  the  altitude. 

The  distribution  of  the  temperature  of  the 
atmosphere  in  a  horizontal  direction  as  ordinarily 
measured  has  reference  merely  to  the  tempera- 
ture of  the  lowest  stratum.  Unlike  the  barome- 
ter which  gives  us  the  sum  total  of  the  pressures 
of  the  superincumbent  layers,  a  thermometer  near 
sea-level  simply  gives  us  the  temperature  of  the 
particular  stratum  in  which  it  lies.  The  magni- 
tude of  the  daily  and  seasonal  changes  vary  ac- 
cording as  the  locality  is  continental  or  maritime, 
and  its  soil  is  dry  and  rocky  or  damp  and  alluvial, 
and  the  average  itself  depends  largely  on  these 
and  other  conditions  besides  mere  latitude. 

The  general  distribution  however  shews  de- 
cidedly that  latitude  is  one  of  the  principal 
causes  which  affect  the  mean  annual  tempera- 
ture. The  map  (fig.  7)  shews  the  distribution  of 
the  heat  in  the  lowest  atmospheric  stratum  over 
the  earth's  surface  on  the  average  of  the  year, 
by  lines  of  equal  average  temperature  (isother- 
mals).  The  principal  points  to  notice  are  the 
widening  out  of  the  area  between  the  isothermals 
of  80°  over  the  land  areas  and  the  contraction 
that  takes  place  over  the  seas,  particularly  the 
Atlantic.  Also  that  wherever  a  marked  dip  of 
the  line,  particularly  that  of  70°,  occurs  toward 
the  pole  over  the  land,  an  equally  marked  dip  of 
the  line  occurs  i'n  the  opposite  direction  close 
alongside. 

This  is  specially   visible  in  California,  Peru, 


THE   TEMPERATURE   OF   THE   ATMOSPHERE. 


45 


46     THE   STORY  OF   THE   EARTH'S   ATMOSPHERE. 

and  in  S.  W.  Africa,  and  is  plainly  due  to  the 
known  existence  of  cold  marine  currents  flowing 
along  these  several  coasts  towards  the  Equator. 
So  far  as  the  British  Islands  are  concerned  it  is 
equally  plain  that  the  northward  flow  of  the 
gulf-stream  of  the  Atlantic  raises  the  isotherm 
of  50°  F.  which  normally  belongs  to  latitude  40° 
and  passes  through  Nippon  (Japan)  ten  degrees 
further  north  so  as  to  make  it  pass  through  Lon- 
don. We  thus  get  in  these  islands  an  atmosphere 
artificially  heated  up  about  10  degrees  (the  iso- 
thermal of  40°  F.  properly  belongs  to  our  lati- 
tude) more  than  we  are  entitled  to  by  our  latitude. 
In  like  manner  Peru  no  doubt  enjoys  several 
degrees  less  heat  than  it  would  otherwise  have 
owing  to  the  cool  Antarctic  stream  (Humboldt's 
current)  which  flows  up  its  coast  and  cools  the 
lower  atmosphere. 

The  area  of  greatest  heat  is  where  the  largest 
land  masses  lie  near  the  Equator,  and  of  greatest 
cold  where  the  largest  land  masses,  such  as  N. 
Asia  and  N.  America,  lie  nearest  the  Pole. 

The  reason  for  this  is  too  important  to  be 
omitted. 

If  the  same  amount  of  sun  heat  falls  upon  an 
equal  area  of  land  and  water  it  raises  the  temperature 
of  the  former  four  or  five  times  as  much  as  that  of 
the  latter. 

Less  heat  energy  is  spent  in  agitating  the 
molecules  of  dry  earth  than  those  of  water. 
Consequently  its  effects  are  more  patent.  In  the 
case  of  water  the  heat  energy  is  not  lost,  no  en- 
ergy ever  is  in  this  Universe — but  it  is  more  latent 
and  the  expressed  temperature  is  less.  The  at- 
mosphere is  more  readily  heated  by  the  radiation 
from  the  hotter  (as  we  say)  earth  than  the  cooler 


THE  TEMPERATURE  OF  THE  ATMOSPHERE.  47 

water.  Consequently  the  lowest  stratum  over 
the  land  areas  under  the  more  direct  sun  near  the 
Equator  exhibits  a  generally  higher  temperature 
than  that  which  lies  over  oceans  in  the  same 
latitude.  Since  heating  and  cooling  are  recipro- 
cal operations  it  is  easy  to  see  that  the  reverse 
applies  to  the  temperature  over  polar  seas  and 
continents. 

The  migration  of  the  sun  north  and  south  of 
the  equator  by  reason  of  the  inclination  of  the 


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Distribution  of  atmospheric 
erature  in  latitude,  for  Jan- 
July,  and  the  year. 

axis    to    the    plane   i 
the  centres  of    the   su 
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greatest  heat  north  and  south  of  the  geographi- 
cal equator.  While  the  sun  shifts  from  23^°  N.  to 
23^°  S.,  however,  the  central  line  of  greatest  heat 
(or  heat-equator)  migrates  to  a  less  amount,  par- 
ticularly over  the  oceans.  On  the  Pacific  it  moves 
only  15°  to  20°  in  latitude.  On  the  Atlantic  still 
less.  On  the  land  it  shifts  as  much  as  43°  in 
Africa  and  50°  in  America,  while  in  India  it  runs 
4 


48  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

up  to  the  deserts  of  Persia  in  latitude  33°  N.  in 
the  summer,  and  down  to  only  10°  S.  in  the  win- 
ter, because  there  are  no  southern  lands  to  at- 
tract it  further. 

The  general  distribution  in  latitude  and  mi- 
gration of  the  temperature  may  be  best  seen  in 
the  accompanying  diagram  (fig.  8)  plotted  out 
from  the  means  given  by  the  late  Mr.  Ferrel  of 
the  American  Weather  Department. 

The  position  of  the  thermal  equator  to  the 
north  of  the  line  and  the  greater  annual  range  of 
temperature  in  the  northern  hemisphere  are 
plainly  visible. 

If  we  were  to  undertake  a  balloon  voyage 
round  the  world  at  an  average  altitude  of  about 
5000  feet  we  should  find  very  few  signs  of  this  pe- 
culiar distribution  which  prevails  near  the  surface. 

The  temperature  over  the  equator  would  be 
about  64°  F.,  an  agreeable  summer  temperature 
in  England,  and  though  if  we  preserved  the  same 
elevation,  the  temperature  would  descend  to  about 
34°  over  London,  the  seasonal  and  daily  changes 
would  be  very  much  less  conspicuous  than  near 
the  surface. 

By  means  of  thermometers  and  thermographs 
the  temperature  of  the  atmosphere  near  the 
surface  is  read  at  certain  hours  or  recorded  con- 
tinuously, and  for  various  purposes  particular 
attention  is  paid  to  its  maximum,  minimum,  and 
average,  either  for  a  day  of  twenty-four  hours,  a 
month  of  30  days,  a  year,  or  a  series  of  years. 

Where  it  is  difficult  to  have  continuous  hourly 
readings  taken,  three  hours  are  chosen,  which, 
when  combined  in  a  simple  manner,  give  a  value 
which  is  found  by  experience  to  closely  approxi- 
mate to  the  average  of  the  day. 


THE  TEMPERATURE  OF  THE  ATMOSPHERE.  49 

Thus,  the  average  of  a  single  reading  at  9  A.  M. 
gives  a  very  close  approximation  to  the  mean  of 
the  twenty-four  hours.  Or  we  may  add  the  read- 
ings at  six,  fourteen,  and  twenty-two  hours  and 
divide  by  three,  or  take  the  lowest  and  highest 
readings  and  divide  by  two.  Where  the  self-re- 
cording thermograph  is  employed,  the  mean  can 
be  found  by  measuring  the  area  traced  out  by  the 
recording  pencil  and  bisecting  it. 

The  maximum  and  minimum  in  this  case 
correspond  to  the  highest  and  lowest  points  of 
the  curve  traced  out,  but  usually  they  are  meas- 
ured by  separate  maximum  and  minimum  ther- 
mometers. 

Apart  from  the  general  distribution  of  its 
mean  annual  values  shown  in  Buchan's  isothermal 
chart,  the  temperature  of  the  lowest  air  stratum 
and  proportionally  of  those  lying  above  it,  is 
subject  to  regularly  recurring  daily  and  seasonal 
oscillations.  These  two  series  of  changes  are 
so  important  in  their  relation  to  our  general 
comfort  and  welfare  that  it  is  of  the  highest  in- 
terest for  us  to  know  whether  they  exhibit  any 
signs  of  progressive  change  in  obedience  to  law 
as  we  vary  our  position  on  the  earth.  As  a 
general  rule  we  find  the  greatest  ranges  of  the 
temperature  of  the  lowest  atmospheric  stratum 
between  day  and  night  occur  in  the  driest  parts 
of  the  earth,  in  the  interior  of  continents,  such  as 
the  Sahara,  Arabia,  Gobi  desert,  Rajputana,  Colo- 
rado, etc.,  where  it  often  amounts  to  40°  F.,  and 
the  smallest  ranges  in  small  oceanic  islands,  such 
as  Honolulu,  Kerguelen,  Madeira,  Bermuda, 
where  it  is  as  small  as  5°  F. 

In  India,  which  presents  the  greatest  contrasts 
of  dry  interior  and  moist  coast  in  the  world,  we 


50  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

have  daily  ranges  of  30°  to  40°  in  the  Punjab,  20° 
to  30°  in  the  Central  Provinces,  16°  at  Calcutta, 
8°  at  Bombay,  and  6°  at  Galle  in  Ceylon.  The 
daily  range  also  decreases  generally  from  the 
Equator  to  the  poles  when  we  take  places  at  the 
same  distance  from  the  sea.  Thus,  while  it  is  n 
degrees  at  Colombo,  it  sinks  down  to  an  average 
of  only  3  degrees  at  Suchta  Bay,  in  latitude  73° 
N.  Even  at  St.  Petersburg,  surrounded  by  large 
continents,  it  is  only  7°. 

The  reason  for  this  is  simple.  The  changes 
in  the  solar  altitude  between  sunrise  and  sunset 
are  manifestly  more  marked  where  the  sun  rises 
higher  in  the  sky  than  where  its  path  is  at  a 
small  inclination  to  the  horizon  all  day,  while  at 
the  Poles,  where  it  takes  a  year  to  rise  and  set 
once,  the  daily  variation  entirely  vanishes. 

The  diurnal  range  of  the  temperature  of  the  air 
also  diminishes  with  elevation  above  the  sea  level. 

Thus  in  the  N.  W.  Himalaya,  while  the  mean 
daily  ranges  at  Mussourie  and  Ranikhet  at  6000 
feet  above  the  sea  are  only  13°  and  15°  F.,  the 
ranges  at  Bareilly  and  Roorkee  on  the  adjacent 
plains  are  23°  and  24°  F.  .  The  reason  is  obvious 
if  we  remember  that  the  heat  received  during  the 
day  is  more  absorbed  by  the  denser  air  near  sea- 
level  than  the  rarer  air  on  the  mountains.  Con- 
sequently, since  the  heat  which  falls  on  the  moun- 
tain-top is  more  freely  radiated  back  into  space, 
the  air  over  the  mountains  is  less  expanded  than 
that  over  the  adjacent  plains. 

During  the  day  since  the  air  over  the  latter 
expands  upwards  about  13  feet  for  every  degree 
F.  the  temperature  of  the  entire  mass  up  to  6000 
rises.  Meanwhile  since  the  mountain  cannot  ex- 
pand the  air  over  it  remains  sensibly  stationary. 


THE  TEMPERATURE  OF  THE  ATMOSPHERE.  51 

In  consequence  a  downflow  takes  place  towards 
the  mountain  somewhat  like  the  sea-breeze  towards 
a  coast  which  brings  with  it  the  cooler  tempera- 
ture in  the  free  air  at  the  same  level  and  so  cools 
that  on  the  mountain. 

At  night  when  the  mountain  which  is  a  good 
radiator  cools  down  rapidly  and  chills  the  air 
which  lies  on  it,  this  air  by  reason  of  its  increased 
density  slides  down  the  mountain  side  and  its 
place  being  taken  by  the  adjacent  less  cooled  air, 
the  temperature  is  again  prevented  from  descend- 
ing too  low. 

In  valleys  on  the  other  hand  even  at  high  alti- 
tudes, the  contrary  conditions  take  place. 

By  day,  owing  to  the  greater  perfection  of  the 
atmospheric  enclosure,  the  sun's  heat  is  more 
effective  in  warming  up  the  lower  stratum  of  air, 
while  at  night  the  chilled  air  from  the  surround- 
ing mountain  tops  descends  into  the  valley  and 
increases  the  cold.  Hence,  at  Leh  in  Thibet, 
which  lies  in  a  valley  at  11,500  feet  elevation  the 
daily  range  is  as  high  as  29  degrees.  On  a  smaller 
scale  it  is  practically  recognised  that  frosts  prevail 
more  in  valleys  than  on  hill  tops. 

The  atmosphere  and  the  ocean  thus  exert  a 
similar  tendency  in  reducing  temperature  ranges, 
and  the  man  who  builds  his  house  on  a  hill  and 
so  rises  into  the  atmosphere,  enjoys  similar  ad- 
vantages to  the  one  who  takes  up  his  residence 
on  the  sea-coast  or  an  island.  In  both  cases  ex- 
tremes of  temperature  are  avoided. 

The  temperature  is  lowest  as  a  rule  on  land 
shortly  before  sunrise.  In  tropical  countries, 
such  as  India,  where  it  occurs  only  just  a  few 
minutes  before  sunrise,  it  is  often  the  only  toler- 
able moment  of  the  24  hours. 


52  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

The  highest  temperature  occurs  nearly  every- 
where on  land  between  2  and  3  p.  M.,  but  alters 
according  to  season. 

The  greatest  changes  in  the  times  at  which 
the  daily  temperature  variation  reaches  its  high- 
est and  lowest  points  are  related  to  the  position 
of  the  place  as  regards  the  ocean. 

Put  briefly,  the  drier  and  more  inland  or  con- 
tinentally  the  place  is  situated,  the  later  will  be 
the  epochs,  while  out  in  the  open  ocean  the  mid- 
day maximum  occurs  soon  after  noon  and  the 
morning  minimum  as  early  as  4  A.  M. 

The  annual  range  of  temperature,  or  in  other 
words  the  difference  between  the  average  tem- 
peratures of  the  hottest  and  coldest  months,  in 
contrast  to  the  diurnal  range,  increases  from  the 
equator,  where  it  is  least,  to  the  poles.  It  also 
increases  with  the  distance  from  the  coast.  Thus 
while  it  is  only  3^°  at  Colombo,  it  is  11°  at  Bom- 
bay, 21°  at  Calcutta,  and  from  30°  to  40°  in  N.  W. 
India. 

The  accompanying  map,  in  which  the  lines  of 
equal  annual  range  of  5°,  10°,  20°,  30°,  etc.,  are 
drawn,  shews  at  a  glance  its  general  distribution 
over  the  earth,  from  which  it  is  plain  that  while  it 
is  least  over  a  broad  belt  surrounding  the  equator, 
it  reaches  its  highest  values  in  the  poleward  cen- 
tres of  the  continents. 

In  England  the  range  of  temperature  between 
summer  and  winter  is  about  20  degrees.  In 
Honolulu  it  is  only  5  degrees,  as  near  the  equator, 
while  at  Werkojansk  in  North  Eastern  Siberia  it 
amounts  to  no  less  than  120  degrees.  The  man 
who  boasts  he  can  wear  the  same  coat  summer 
and  winter  through  would  have  to  change  his 
habits  in  that  district.  There  are  several  re- 


THE  TEMPERATURE  OF  THE  ATMOSPHERE.  53 

markable  features  exhibited  on  this  map.  One 
is  that  in  all  the  northern  continents  the  position 
of  the  greatest  annual  temperature  range  is  to 
the  east  of  their  geographical  centres.  This  is 
chiefly  owing  to  the  influence  of  the  warm  cur- 
rents which  bathe  their  western  shores  and  the 
accompanying  wind  currents  which  carry  the 


FIG.  9. 

moderating  effect  of  the  ocean  over  a  large  part 
of  their  western  interiors.  Western  Europe  is 
peculiarly  favoured  in  this  respect. 

Also  the  generally  small  range  over  the  larger 
oceans  which  is  due  to  the  slow  response  of  masses 
of  water  to  the  seasonal  changes  in  the  amount 
of  solar  heat  falling  on  it,  a  point  which  has  al- 
ready been  attended  to.  As  a  result,  the  ranges 
over  the  southern  hemisphere  which  is  mostly 
water  are  uniformly  small.  In  New  Zealand,  for 
example,  December  and  June  differ  by  only  10 
degrees. 

Besides  the  diurnal  and  annual  ranges  of  tern- 


54  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

perature  it  is  found  that  there  are  slow  periodic 
changes  of  a  small  amount  in  the  mean  tempera- 
tures of  the  year,  in  correspondence  with  the 
changes  in  the  number  and  area  of  sunspots 
which  recur  about  every  eleven  years.  Whatever 
may  be  the  exact  cause,  whether  an  increase  or 
decrease  of  solar  radiation  corresponding  to  a 
great  spot  manifestation,  the  effects  have  been 
proved  through  the  labours  of  Prof.  Piazzi  Smyth, 
Dr.  Stone,  Dr.  Koppen  of  Hamburg,  and  Prof.  Fritz 
of  Zurich,  to  be  visible  in  the  temperature  of  the 
earth's  atmosphere. 

In  years  grouped  round  those  of  fewest  spots, 
such  as  1811,  -23,  -34,  -43,  -56,  -67,  -77,  -88,  the 
temperature  is  highest,  and  in  those  similarly 
grouped  round  those  of  most  spots  such  as  1860, 
-71,  -83,  -93,  it  is  lower  than  the  average.  The 
effect  is  most  noticeable  in  the  tropics.  For  ex- 
ample, in  India,  the  difference  between  the  tem- 
perature at  the  two  epochs  varies  from  i  to  2 
degrees  on  the  mean  of  the  year. 

A  similar  periodic  change  is  found  to  prevail 
in  conditions  which  depend  upon  air  temperature, 
such  as  fruit-harvests,  vintages,  rainfall,  glacier 
extension,  storms,  cloud  proportion,  etc.,  while 
the  late  Professor  Jevons  endeavoured  to  shew 
that  even  commercial  panics  were  brought  about 
periodically  through  the  medium  of  such  indirect 
consequences. 

Though  there  is  much  scepticism  as  to  the 
quantity  of  the  temperature  effect  being  of  such 
importance  as  to  bring  about  panics  through  bad 
harvests,  there  is  no  doubt  that  the  condition  of 
the  sun  affects  our  atmosphere  in  some  peculiar 
way  not  only  in  regard  to  temperature,  but  also 
magnetically,  since  the  appearance  of  the  aurora 


THE  TEMPERATURE  OF  THE  ATMOSPHERE.  55 

is  admitted  by  those  who  dispute  the  heating  ef- 
fect to  be  closely  connected  with  the  presence  of 
sunspots  and  other  forms  of  solar  disturbance. 

This  periodic  oscillation  of  annual  tempera- 
ture does  not  of  course  involve  any  steady  pro- 
gressive change  in  the  temperature  of  the  atmos- 
phere. In  fact,  when  some  years  ago  the  people 
of  Paris  were  temporarily  afraid  that  their  climate 
was  changing,  the  astronomer  Arago  proved  to 


FIG.  10. 

their  satisfaction,  by  a  recourse  to  statistics,  that 
the  temperature  of  Paris  had  not  sensibly  changed 
for  100  years,  and  within  historical  periods  there 
does  not  seem  to  be  any  evidence  that  the  tem- 
perature anywhere  is  sensibly  changing  perma- 
nently one  way  or  another. 

We  will  now  pass  on  to  consider  the  circum- 
stances that  attend  a  local  accession  of  heat  over 
land  and  water  and  the  primary  effects  which  it 
produces. 

Beginning  with  any  area  on  a  small  scale. 
Let  fig.  (10)  represent  a  vertical  section  of  the 
atmosphere  and  let  the  dotted  lines  represent 


56  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

lines  of  equal  barometric  pressure  beginning  with 
30  inches  at  the  earth's  surface,  and  let  us  sup- 
pose that  the  temperature  of  the  central  region 
is  raised  by  a  certain  amount. 

All  the  air  thus  warmed  will  expand.  The 
column  H.A.  will  expand  to  height  H.B.,  and  as 
each  layer  will  expand  all  the  way  up,  the  sur- 
face of  the  top  layer  will  be  most  raised.  Con- 
sequently there  will  be  a  flow  outwards  of  the 
raised  up  air  down  the  slopes  marked  by  the 
thick  lines  toward  the  neighbouring  air  of  the 
same  pressure,  which,  not  being  expanded,  lies  at 
a  lower  level. 

The  outflow  will  be  greatest  in  the  highest 
layer  since  it  is  the  most  raised  (the  increase  is 


denoted    by    the    varying    size    of   the    arrows). 
Meanwhile  the  loss  of  air  above  will  lessen  the 


THE  TEMPERATURE  OF  THE  ATMOSPHERE.  57 

pressure  on  the  earth's  surface  near  the  centre  of 
the  area.  Consequently  the  surrounding  air  will 
flow  in  towards  this  centre  chiefly  in  the  lowest 
layer,  and  the  action  having  once  started  will 
continue  so  long  as  the  central  area  is  more 
heated  than  the  neighbourhood. 

We  have  already  noticed  that  where  sun  heat 
falls  upon  land  it  heats  it  up  more  readily  than 
water.  Therefore  particularly  in  the  case  of  an 
island  lying  in  a  tropical  sea  where  the  sun  is 
powerful  the  above  action  takes  place  as  in  fig. 
(n)  and  we  have  the  phenomenon  known  as  the 
local  sea  breeze.  When  the  sun  disappears  at 
night  the  action  is  precisely  reversed,  and  the 
air  near  the  surface  flows  outwards  as  the  land 
breeze,  while  above  a  certain  height,  which  in 
local  cases  is  often  as  low  as  1000  feet,  the  air 
streams  in  over  the  rapidly  cooling  land. 


FIG.  12. 


After  the  action  has  once  started  things  ar- 
range themselves  as  in  fig.  (12)  where  the  lower 
curved  lines  represent  the  depression  caused  by 
the  loss  of  air  which  has  flowed  outwards  above, 
and  where  NN1  represents  where  the  tendency 


5  8  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

to  flow  in  and  out  neutralise  each  other  and  there 
is  a  plane  of  no  motion  called  sometimes  the 
neutral  plane. 

The  above  action  involves  a  certain  amount 
of  upward  motion  of  the  air  over  the  central  part 
of  the  heated  area,  and  a  corresponding  down- 
ward motion  over  the  surrounding  cooler  area,  but 
these  movements  are  evidently  much  smaller  than 
the  horizontal  outflow  and  inflow.  The  same 
action  also  explains  the  origin  of  the  manifest 
monsoons  of  Asia  and  Australia,  where  in  the 
summer  season  the  air  blows  more  or  less  towards 
a  heated  land  area,  and  in  the  winter  from  it 
towards  the  surrounding  sea. 

It  also  accounts  in  part  for  the  annual  changes 
in  the  barometric  pressure  over  large  areas,  espe- 
cially the  low  pressures  in  the  middle  of  the  larger 
continents  like  Asia  and  America  during  the  sum- 
mer, and  the  corresponding  very  high  pressures 
at  the  opposite  season. 

Unless  some  such  system  of  rise  and  overflow 
over  the  hotter  areas  and  sinking  and  underflow 
over  the  cooler  areas  took  place,  the  barometer 
would  record  a  steady  pressure  over  both  areas, 
and  if  we  ascended  over  the  more  heated  area  we 
should  find  the  pressure  greater  than  at  the  same 
level  over  the  cooled  area,  because  the  air  being 
more  expanded  vertically,  there  would  be  more 
top  cover  so  to  speak  over  our  heads. 

As  a  matter  of  fact,  notwithstanding  the  over- 
flow which  relieves  this  state  of  things,  the  pressure 
at  highly  elevated  stations  like  Leh  (11,800  feet) 
north  of  Kashmir,  rises  until  the  beginning  of 
May,  and  only  falls  very  slightly  in  June  and 
July.  Consequently  the  lowering  of  pressure 
which  appears  so  distinctly  over  Southern  Asia  in 


THE  TEMPERATURE  OF  THE  ATMOSPHERE.  59 

the  summer  is  confined  to  the  lower  half  (by  mass) 
of  the  atmosphere,  that  is  to  say  below  18,000 
feet,  at  which  level  the  pressure  is  15  inches. 
Above  this  level  there  is  more  or  less  an  outflow 
in  the  summer  and  an  inflow  in  the  winter. 

A  similar  system  of  land  and  sea  monsoonal 
circulation  exists  everywhere,  only  in  high  lati- 
tudes it  is  ordinarily  masked  by  other  motions  of 
the  air,  introduced  by  the  frequent  passage  of 
cyclones,  and  large  travelling  systems  or  waves 
of  high  and  low  pressure.  Even  along  Western 
Europe  the  winds  blow  more  towards  the  land  in 
summer  and  from  it  in  the  winter. 

Where  a  small  area  on  the  land  or  sea  is 
heated  up  above  its  neighbourhood  we  have  the 
initial  conditions  for  the  formation  of  a  disturb- 
ance of  equilibrium.  In  hot  countries  where  such 
a  condition  is  more  prevalent,  there  may  arise  a 
cyclone,  tornado,  whirlwind,  or  thunder-storm, 
under  different  conditions,  which  will  be  alluded 
to  later  on,  but  in  order  that  there  may  be  intense 
local  action  and  a  real  "  courant  ascendant"  the  air 
must  not  be  merely  gently  lifted  up  and  overflow, 
which  is  the  only  possible  condition  when  it  is 
dry,  but  it  must  be  nearly  saturated  with  vapour, 
in  which  case  it  will  flow  upwards  so  long  as  the 
lowest  stratum  continues  to  supply  damp  air. 
The  part  taken  by  temperature  in  causing  these 
phenomena  will  be  alluded  to  in  a  later  chapter. 

The  present  account  of  the  temperature  of  the 
atmosphere  would  be  incomplete  if  it  omitted  to 
notice  the  transfer  of  heat  from  one  part  of  the 
earth  to  another. 

So  far  we  have  merely  examined  the  heat 
which  falls  locally  or  generally  by  means  of  the 
direct  solar  radiation. 


60  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

The  temperature  over  any  region  is  however 
largely  dependent  on  the  heat  brought  to  it  by 
winds.  When  they  come  from  the  sea  their  tem- 
perature is  modified  by  the  influence  of  the  ocean 
currents,  warm  or  cold,  over  which  they  have 
traveled.  When  they  come  from  the  interior  of  a 
continent,  they  are  usually  hotter  in  the  summer 
and  colder  in  the  winter  than  the  maritime  re- 
gions towards  which  they  advance. 

Thus  in  summer  our  hottest  wind  in  England 
is  the  south-east,  and  the  same  wind  often  ac- 
companies our  most  severe  frosts  in  the  winter. 
The  thermal  effects  of  land  winds  therefore  change 
with  the  season. 

Sea  winds,  especially  where  they  are  connected 
with  ocean  currents,  and  blow  with  some  degree 
of  constancy,  exercise  a  permanent  influence 
upon  the  temperature  of  countries  over  which 
they  prevail.  The  most  marked  warm  sea  winds 
are  felt  on  Western  Europe,  the  Pacific  slope 
north  of  lat.  40°,  and  the  eastern  coast  of  South 
America. 

These  winds  are  not  merely  warm  because 
they  have  accompanied  streams  of  warm  water, 
such  as  the  Gulf  stream  of  the  Atlantic  and  the 
Japan  stream  of  the  Pacific,  but  because  their 
cooling  is  retarded  by  the  latent  heat  set  free  in 
the  condensation  of  the  vapour  they  bring  from 
the  humid  tropics. 

Several  attempts  have  been  made  to  measure 
the  heat  conveyed  by  both  these  streams.  Dr. 
Haughton  of  Dublin  some  years  ago  estimated 
that  these  two  streams  together  carried  one-third 
of  the  total  heat  received  by  the  northern  tropi- 
cal zone  towards  the  middle  latitudes.  Ferrel, 
however,  has  more  recently  shown  that  it  is  more 


THE  TEMPERATURE  OF  THE  ATMOSPHERE.  6l 

probably  one-sixth.  As  we  have  already  seen, 
the  effect  on  England  is  to  raise  the  mean  tem- 
perature nearly  10  degrees  above  what  it  would 
otherwise  be.  Norway  is  raised  as  much  as  16 
degrees,  and  Spitzbergen  19  degrees.  On  the 
other  hand  compensating  cold  currents  and  the 
winds  which  blow  off  them  depress  the  tempera- 
ture of  Eastern  Canada,  northern  China,  western 
South  America,  and  western  South  Africa.  New- 
foundland is  thus  about  10  degrees  colder  than 
the  normal  for  the  latitude.  The  North  China 
coast  about  7  degrees  colder,  and  even  Honolulu, 
in  the  mid  Pacific,  has  its  temperature  reduced  5 
degrees  by  the  return  Japan  stream  cooled  after 
losing  its  heat  up  north. 

The  general  influence  of  the  ocean  currents  in 
reducing  the  difference  which  would  exist  be- 
tween the  temperature  at  the  equator  and  the 
poles,  may  be  inferred  from  the  fact,  that  accord- 
ing to  Ferrel,  if  the  surface  of  the  earth  were  en- 
tirely dry  land,  and  there  were  consequently  no 
transfer  of  heat  by  oceanic  or  atmospheric  cur- 
rents, theoretical  considerations  shew  that  the 
temperature  at  the  equator  would  stand  at  about 
131°  F.,  while  that  at  the  Pole  would  be  108°  be- 
low Zero. 

Observations,  however,  shew  that  the  mean 
temperature  day  and  night  at  the  Equator  is 
about  80°  F.,  while  that  at  the  Pole  is  only  o°  F. 
or  Zero.  Consequently  the  effect  of  the  circula- 
tion of  the  ocean  and  the  atmosphere  together  is 
to  depress  the  temperature  at  the  Equator  about 
50  degrees  and  raise  that  at  the  Pole  no  less  than 
100  degrees,  and  in  this  manner  render  the  earth 
generally  fit  for  human  habitation,  since,  if  such 
extremes  as  those  mentioned  prevailed,  man 


62  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

would  have  been  forced  to  inhabit  a  very  con- 
stricted zone  in  middle  latitudes.  In  like  manner 
were  the  earth  deprived  of  its  atmosphere  the 
mean  temperature  at  the  Equator  would  be  94 
degrees  below  zero  F.,  while  that  at  the  poles 
would  be  328  degrees  below  zero  F.,  and  the 
mean  temperature  of  the  whole  globe  138  degrees 
below  zero  F., — a  terrible  frost.  In  fact,  even  if 
it  were  possible  to  do  without  air  the  human 
species  as  at  present  constituted  would  in  such  an 
event  be  quite  unable  to  exist.  With  the  protec- 
tion of  an  atmosphere  the  average  temperature  of 
the  earth,  or  more  correctly  of  the  lowest  stratum 
of  the  atmosphere  is  about  60°  F.  which  is  re- 
garded as  the  most  delightful  that  can  be  enjoyed. 
So  much  do  we  owe  to  the  invisible  envelope  of 
atmospheric  air,  which  otherwise  appears  to  con- 
stitute such  a  flimsy  blanket  between  us  and  the 
terrible  cold  of  stellar  space. 

Extreme  local  temperatures  are  due  to  the 
concurrence  of  accidental  causes  tending  to  raise 
or  lower  the  temperature,  such  as  the  passage  of 
storms,  prevalence  of  winds  from  north  or  south, 
long  continued  clear  weather,  combined  with 
those  of  more  regular  incidence.  Extremely 
high  temperature  will  generally  occur  in  these 
latitudes  soon  after  noon  in  July  and  August, 
and  extremely  low  ones  early  in  the  morning  in 
January  or  February.  Occasionally,  however, 
the  epochs  are  considerably  displaced. 

The  highest  temperatures  on  the  earth  usually 
occur  in  India,  the  Red  Sea,  the  Persian  Gulf,  and 
Australia.  Thus  in  the  centre  of  the  Sahara,  130 
degrees  has  been  recorded.  At  Jacobabad  in  the 
Sind  desert,  the  temperature  frequently  rises  over 
120°  F.  and  even  in  New  South  Wales,  120°  and 


THE  TEMPERATURE  OF  THE  ATMOSPHERE.  63 

121°  have  been  recorded  at  Bourke  and  Denili- 
quin.  In  February,  1896,  a  temperature  of  108 
degrees  was  recorded  at  Sydney,  due  to  a  remark- 
able prevalence  of  dry  N.  W.  winds  blowing  over 
it  from  the  interior. 

Paris  has  only  once  reached  106  degrees  and 
London  has  seldom  recorded  anything  over  96°. 

The  coldest  temperatures  are  found  not  at  the 
poles  themselves,  where  the  water  circulation 
tends  to  bring  heat  from  the  equator  but  in  the 
north-east  of  Siberia  and  north-east  America. 

Werkojansk  is  the  coldest  place  in  the  world. 
In  January  the  mean  temperature  there  is  55°  F. 
below  zero  while  all  through  the  year  the  temper- 
ature is  only  5  degrees  above  zero. 

During  arctic  expeditions,  the  Alert  and  Dis- 
covery experienced  73°  below  zero,  while  Capt. 
Nares  once  saw  the  thermometer  descend  to  84° 
F.  below  zero. 

Of  recent  years,  a  great  extension  of  our 
knowledge  of  the  phenomena  of  the  atmosphere 
has  been  made  by  the  application  of  what  is 
known  as  thermo-dynamics. 

Prof.  Bezold  of  Berlin,  the  late  Mr.  Ferrel  of 
Washington,  Dr.  Hann  of  Vienna,  and  others  have 
cleared  away  much  of  the  loose  and  misty  reason- 
ings which  characterize  the  work  of  their  prede- 
cessors, but  the  subject  is  too  difficult  and  tech- 
nical to  be  alluded  to  here.  A  few  of  the  leading 
ideas  however  will  be  briefly  touched  upon  when 
some  of  the  particular  atmospheric  phenomena 
are  being  described  further  on. 


64  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 


CHAPTER   V. 

THE    GENERAL    CIRCULATION    OF    THE 
ATMOSPHERE. 

THE  "  Story  of  the  Winds  "  is  interesting  and 
important  enough  to  form  the  subject  of  a  sepa- 
rate volume  and  within  the  compass  of  one  which 
endeavours  to  cover  the  varied  phenomena  of  the 
atmosphere  generally,  only  the  more  salient 
points  in  connection  with  atmospheric  motion  can 
be  reviewed.  In  these  latter  days,  in  spite  of  the 
old  saying  that  "  the  wind  bloweth  where  it  list- 
eth  "  and  the  manifest  and  apparently  capricious 
changes  which  characterize  its  behaviour  in  these 
midway  latitudes,  we  know  that  there  exists  an 
independent  dominating  scheme  of  general  circu- 
lation between  the  poles  and  the  equator.  This 
scheme  results  from  the  action  of  nearly  perma- 
nent differences  of  temperature  between  these 
points  in  combination  with  certain  mechanical 
laws  resulting  from  the  shape  of  the  earth  and  its 
rotation  on  its  axis. 

In  former  days  many  guesses  were  made  more 
or  less  at  variance  with  both  facts  and  theory. 
Even  Maury's  fascinating  attempt  in  1855  to 
weave  observation  into  a  connected  system,  failed 
owing  to  the  imperfect  knowledge  existing  at  that 
time  of  the  winds  of  the  entire  globe  as  well  as  of 
the  true  laws  which  operated. 

The  earliest  attempt  at  any  rational  scheme  of 
accounting  for  the  more  obvious  features  of  the 
general  circulation  appears  to  have  been  made  in 
1735  by  Hadley. 

The  regularity  of  the  "  trade  winds  "  was  then 


GENERAL  CIRCULATION  OF  THE  ATMOSPHERE.    65 

attracting  the  attention  of  scientists,  and  in  a 
short  paper  in  the  Philosophical  Transactions, 
Hadley  advanced  a  theory  to  account  for  this 
which  sounded  so  plausible,  that  for  over  a  cen- 
tury it  remained  unquestioned. 

Hadley's  theory  in  brief  was,  that  owing  to  the 
general  difference  of  temperatures  between  the 
polar  and  equatorial  regions,  a  motion  of  the  air 
took  place  similar  to  that  just  described  in  the 
last  chapter,  in  the  lower  strata  towards  the  zone 
of  greatest  heat,  while  the  easterly  *  direction  of 
the  trades  was  attributed  to  the  fact  that  as  the 
air  continually  arrived  at  parallels  where  the 
earth's  surface  moved  faster  eastwards  than  the 
part  it  had  just  left,  it  tended  continually  to  lag 
behind  in  a  westward  direction,  and  so  appear  to 
blow  partly  from  the  east.  Hence  it  became  the 
north-east  trade  on  the  northern  and  the  south- 
east trade  on  the  southern  side  of  the  equator. 
Carried  to  its  logical  conclusions  Hadley's  theory 
would  require  the  trades  to  blow  all  the  way 
from  the  poles  to  the  equator,  the  return  current 
being  confined  almost  entirely  to  the  upper  air. 

Moreover  the  highest  pressure  as  measured 
by  the  height  of  the  mercury  column  in  the 
barometer  air  balance,  should  be  found  at  or  near 
the  poles. 

As  a  matter  of  fact,  however,  it  was  found 
that  neither  of  these  circumstances  took  place. 

The  trades  extended  no  further  than  latitudes 
30  degrees  N.  and  S.  of  the  equator,  the  pressure 
at  the  poles,  especially  the  south  pole  was  per- 
manently lower  than  at  the  equator  (about  £ths 
of  an  inch  of  mercury)  while  the  highest  pressure 

*  From  the  East. 


66  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE 

was  found  to  occupy  two  belts  between  30°  and 
40°  N.  and  S.  of  the  equator. 

Obviously  therefore  there  was  something  radi- 
cally wrong  with  Hadley's  theory. 

In  1856  Mr.  Ferrel,  afterwards  Professor  in 
the  United  States  Weather  bureau,  tackled  the 
subject  and  found  out  that  Hadley  had  entirely 
overlooked  the  fact  that  the  earth  is  a  sphere. 

In  consequence  his  theory  contained  two  seri- 
ous errors,  one  of  which  was  that  only  air  moving 
north  and  south  was  deflected  by  the  earth's  rota- 
tion, while  that  moving  in  any  other  direction  re- 
mained unchanged. 

The  only  circumstances  to  which  Hadley's 
theory  could  possibly  apply  would  involve  the 
supposition  that  the  earth  was  a  perfectly  flat 
plane  composed  of  separate  planks  parallel  to  a 
straight  line  equator.  Also  that  these  planks 
moved  along  with  different  speeds  beginning 
with  1000  miles  at  the  equator  and  gradually 
decreasing  to  about  850  miles  at  latitude  30°, 
manifestly  a  very  different  affair  from  a  spherical 
surface  like  that  of  the  earth. 

Some  few  years  before  Ferrel  approached  the 
question,  the  eminent  French  mathematician 
Poisson  in  1837  read  a  paper  before  the  Paris 
Academy,  in  which  he  demonstrated  that  when 
a  freely  moving  body  passes  over  the  earth's 
surface  in  any  direction,  the  effect  of  the  earth's 
rotation  is  to  cause  it  to  deviate  (not  lag)  to  the 
right  of  its  path  in  the  northern  hemisphere,  and 
to  the  left  in  the  southern. 

Employing  the  same  reasoning  as  Poisson, 
but  applying  it  to  masses  of  air  instead  of  solid 
bodies  Ferrel  gradually  built  up  a  satisfactory 
explanation  of  the  general  circulation,  and  with 


GENERAL  CIRCULATION  OF  THE  ATMOSPHERE.    67 

the  help  of  suitable  modifications,  applied  the 
same  principle  to  explain  the  leading  features  of 
cyclones  and  tornadoes. 

In  general,  if  a  mass  of  air  initially  tends  to 
move  on  a  rotating  sphere  toward  a  certain  point, 
impelled  in  the  first  instance  by  a  difference  of 
density  or  pressure,  it  tends  to  move  continually 
to  the  right  when  looked 
at  from  a  point  above  the 
N.  pole  of  rotation  and 
unless  prevented  from  do- 
ing so  by  any  extra  force 
resisting  such  motion, 
would  continue  to  devi- 
ate until  it  had  turned 
through  a  complete  circle 
thus  fig.  (13). 

Suppose  a   particle  of 
air  at  A  starts  to   move  FIG.  13. 

towards    B.      Instead    of 

moving  in  the  straight  line  AB,  it  will  tend  to 
move  in  the  curve  AC,  and  if  it  is  very  near  the 
pole  it  will  eventually  complete  a  circle*  as  above 
in  12  hours,  the  size  depending  on  its  velocity. 
Thus  for  a  speed  of — 

Radius  of  inertia  circle 

in  miles. 

20  miles  an  hour  .  .  .  77 

10    "          "       .  .  .  .  38 

5     "  .  .  .  .  19 

We  have  the  corresponding  values  of  what  is 
termed  the  curve  of  inertia. 

Prof.  Davis  of  Harvard  has  suggested  a  very 

*  In  other  latitudes  the  inertia  curve  as  it  is  termed  is 
more  like  the  series  of  loops  cut  by  a  skater  restricted  be- 
tween certain  limits  of  latitude.  At  the  equator  it  vanishes. 


68     THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

pretty  experiment  which  can  be  performed  by 
any  one  who  wishes  to  have  visual  evidence  of 
the  existence  of  this  inertia  curve. 

Take  a  small  circular  table,  and  lay  a  sheet 
of  white  paper  over  it.  Then  take  a  marble  and 
dip  it  in  ink  and  lay  it  near  the  centre  of  the 
table.  Next  tilt  the  table  slightly  so  as  to  give 
the  marble  a  slight  motion  towards  the  edge,  and 
at  the  same  time  rotate  the  table  about  its  centre. 
Then  the  marble  will  be  found  to  trace  out  in  ink 
a  curved  line  on  the  paper  which  will  fairly  rep- 
resent the  inertia  curve  or  the  curve  of  successive 
deviations  from  a  straight  line  by  which  the  par- 
ticle through  its  inertia  (or  laziness)  is  unable  to 
accommodate  itself  to  the  varying  motion  of  the 
parts  over  which  it  rolls. 

In  whatever  direction  the  table  is  tilted  the 
curve  will  still  be  traced  out,  the  curvature  sharp- 
ening with  increased  rotation  of  the  table  (analo- 
gous to  increased  latitude)  and  lessening  with  in- 
creased tilt  by  which  the  velocity  of  the  particle 
is  augmented. 

This  tendency  of  air  to  move  to  the  right  of 
its  original  direction  of  motion,  is  what  really  ac- 
counts for  the  development  of  those  permanent 
or  rapidly  changing  differences  of  barometric 
pressure  which  accompany  the  large  general  or 
small  particular  air  motions.  A  difference  of 
temperature  alone,  for  example,  between  the 
poles  and  equator,  or  between  two  neighbour- 
ing parts  of  the  earth  would  cause  a  very  slight 
alteration  in  the  barometric  pressures,  but  when 
the  air  begins  to  move  in  the  direction  of  the 
lower  pressure  its  tendency  to  push  to  the  right, 
causes  a  squeezing  and  heaping  up  of  air  to  the 
right  of  its  path,  and  a  corresponding  stretching 


GENERAL  CIRCULATION  OF  THE  ATMOSPHERE.    69 

apart  or  lowering  of  density  and  pressure  to  its 
left,  until  the  difference  of  pressures  becomes  great 
enough  to  prevent  its  further  movement  to  the 
right  and  it  moves  in  a  path  regulated  by  these 
joint  tendencies,  thus — 

Path  (curved 

norm  i  ^f  in  which,  air 

area.  I 


Direction, 
in  which  - 
atr  starts  to 
move 

jTMtmiiimuiiiinnnin   7  ~~&* 

Inertia  curve 
into  which 

iff** .....-.,.,-. .,..-.---    -  cu'r  tends  to 

Cold  area  &  deflected 

force 
exerted  by 
pressure  qraaienC 
FIG.  14. 

Amass  of  air  at  the  cold  area  will  tend  initial- 
ly to  move  towards  the  less  dense  warm  air,  but 
once  it  starts  it  tends  to  move  along  the  inertia 
curve.  Eventually  the  high  pressure  (denoted 
by  the  shading)  of  the  heaped  up  air  on  this  side 
exerts  a  force  indicated  by  the  arrow  directed 
towards  the  increased  low  pressure  to  the  left, 
and  finally  the  air  making  a  compromise  moves 
along  a  line  between  the  two,  indicated  by  the 
direction,  labelled  "  Path,"  etc.,  so  that  instead  of 
moving  directly  from  high  to  low  pressure  it  only 
partly  moves  towards  the  latter,  keeping  the  high 
pressure  to  the  right  and  the  low  pressure  to  the 
left  of  its  path.  In  the  southern  hemisphere 
owing  to  the  reversed  point  of  view  right  be- 
comes left  and  the  high  pressure  would  be  to 


70  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

the  left  of  the  path  and  the  low  pressure  to  the 
right. 

This  diagram  will  be  found  to  supply  the  ex- 
planation of  the  general  relations  between  pres- 
sure and  wind,  especially  if  it  is  remembered  that 
where,  as  on  land  and  near  the  surface,  the  air  is 
prevented  by  friction  from  moving  with  freedom, 
the  back  thrust  in  the  opposite  direction  tends  to 
make  the  ultimate  path  point  more  towards  the 
low  pressure,  while  at  sea  and  at  great  altitudes, 
where  friction  is  small,  it  moves  almost  at  right 
angles  to  the  line  joining  the  central  areas  of 
high  and  low  pressures,  or  in  the  technical  lan- 
guage borrowed  from  engineering,  at  right  angles 
to  the  direction  of  the  barometric  gradient. 

Before  alluding  to  Ferrel's  explanation  of  the 
general  circulation  of  air  over  the  globe  on  these 
principles,  let  us  see  what  this  circulation  really 
is  like  from  observation. 

In  the  two  plates  adjoining,  figs.  (15)  and  (16), 
in  which  the  actually  observed  barometric  pres- 
sures and  winds  at  two  opposite  seasons  of  the 
year  are  represented,  it  will  be  noticed  that,  over- 
looking minor  features,  there  is  a  broad  belt  over 
the  equator,  over  which  the  barometric  pressure 
is  about  29.80  inches,  gradually  rising  on  either 
side  to  two  belts  of  high  pressure,  in  latitude 
30°  in  places  reaching  30.2  inches,  and  generally 
.about  0.2  inches  higher  than  over  the  equator. 
Within  this  area,  the  trade  winds  blow  through- 
out the  year  on  each  side  of  the  equator,  except 
over  the  North  Indian  Ocean,  where  in  July  they 
blow  in  towards  an  area  of  excessively  low  pres- 
sure and  high  temperature  as  the  south-west 
monsoon  of  the  Indian  seas,  which  brings  the 
rain,  that  has  made  India  such  a  much  more  fer- 


GENERAL  CIRCULATION  OF  THE  ATMOSPHERE.    71 

tile  and  populous  country  than  the  neighbouring- 
peninsula  of  Arabia.  In  the  map  fig.  (20),  p.  81, 
the  monsoon  winds  are  represented  blowing  over 
India  during  July.  In  January,  the  south-west 
winds  disappear,  and  in  the  general  chart  it  will 
be  seen  that  their  place  is  taken  by  Northerly  or 
North-easterly  winds,  blowing  down  towards  the 
equator,  from  the  large  area  of  high  pressure 
which  at  this  season  spreads  over  the  whole  of 
north-eastern  Asia. 

On  the  polar  sides  of  these  bands  or  nuclei  of 
high  pressure,  it  will  be  observed  that  the  winds 
blow  more  or  less  towards  the  poles,  especially  in 
the  southern  hemisphere. 

The  lines  (isobars)  on  these  maps,  by  which 
the  changes  in  the  distribution  of  the  mean 
monthly  barometric  pressure  is  indicated,  are 
similar  to  the  contour  lines  or  lines  of  equal  ele- 
vation employed  to  represent  the  contour  of  a 
hilly  country.  They  do  not  necessarily  represent 
real  elevations  or  depressions  of  the  atmosphere, 
because  increased  or  decreased  pressure  is  more 
due  to  a  greater  or  less  squeezing  or  density,  than 
to  a  piling  up  of  the  atmosphere  into  absolute 
heaps  and  hollows,  but  since  the  effective  results 
would  be  very  much  the  same  in  either  case,  they 
may  practically  be  considered  as  atmospheric 
contours.  More  correctly,  they  are  the  lines 
along  which  atmospheric  contours  intersect  the 
earth's  surface,  the  pressure  over  which  at  sea 
level,  (about  30  inches),  lies  half  way  up  the  at- 
mospheric slope.  The  accompanying  figure  will 
render  this  clearer. 

The  sloping  lines  marked  30.2,  30,  29.8  etc.. 
represent  sections  of  the  actual  atmospheric  iso- 
bars or  pressure  contours.  B,  C,  D,  points  where 


72  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 


GENERAL  CIRCULATION  OF  THE  ATMOSPHERE.    73 


74  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

these  lines  cut  the  earth's  surface.  The  dotted 
continuations  represent  how  the  lines  would  run 
if  the  atmosphere  took  the  place  of  the  solid 
earth.  The  curved  lines  starting  from  B  and  C 
to  E  and  J?,  denote  the  lines  as  they  occur  on  the 
earth's  surface  or  appear  on  a  plane  chart,  when 
the  contours  curve  round  a  central  area  A,  where 


296 


FIG.  17. 

the  pressure  in  this  case  is  about  29.7  inches.  In 
considering  the  general  circulation,  A  may  be 
taken  to  represent  the  North  or  South  Pole,  in 
which  case  the  diagram  represents  something  like 
what  actually  takes  place.  When  we  are  dealing 
with  particular  motions,  A  would  correspond  with 
the  centre  of  a  cyclone  or  travelling  disturbance. 

Returning  to  our  story,  it  is  plain  from  these 
maps,  that  the  circulation  of  the  atmosphere 
comprises  a  great  deal  more  than  a  mere  system 
of  trade  winds,  blowing  towards  the  equator. 

Any  theory  that  pretended  to  explain  the  en- 
tire system,  would  have  to  account  for  the  pre- 
vailing high  pressures  about  latitude  30°  N.  and 
S.,  and  the  poleward  trend  of  the  wind  on  the 
polar  sides  of  these  atmospheric  sierras. 

Ferrel,  in  his  first  paper  in  1856,  not  only 
shewed  that  Hadley's  theory  was  mathematically 


GENERAL  CIRCULATION  OF  THE  ATMOSPHERE.    75 

incorrect,  but  that  Maury's  fascinating  scheme, 
put  forward  in  his  Physical  Geography  of  the  Sea, 
erred  both  in  fact  and  the  laws  of  physics. 

Reversing  the  usual  procedure  by  which  laws 
are  induced  from  observations  and  starting  with 
a  few  fundamental  principles,  such  as  the  law  of 
deflection  already  noticed,  he  shewed  that  the 
high  pressure  belt  about  30°  and  the  system  of 
poleward  winds  on  the  polar  sides  of  it  were 
necessary  consequences  of  these  principles. 


FIG.  18. 


Ferrel's  final  ideal  chart  of  atmospheric  circu- 
lation  on  the  earth  is  represented  by  fig.  (18) 


76  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

where  the  average  motions  near  the  surface  are 
represented  in  plan  in  the  shaded  circle,  and  in 
vertical  section  at  the  border,  and  though  his  ex- 
planation is  a  complicated  piece  of  mathematical 
reasoning,  the  following  represents  the  pith  of  it 
in  simple  language. 

Assuming  that  the  air  over  the  equatorial  zone 
is  heated  above  that  near  the  pole,  it  expands 
near  the  equator  and  contracts  over  the  pole. 
Consequently  the  uplifted  air  over  the  equator 
tends  to  flow  downhill  as  it  were  towards  the 
poles  and  a  corresponding  flow  near  the  surface 
takes  place  towards  the  equator.  If  the  earth 
did  not  rotate  on  its  axis  the  upper  air  would 
flow  direct  to  the  pole  then  descend  and  return 
towards  the  equator  along  the  surface.  Since 
however  the  earth  does  rotate,  the  upper  air  is 
deflected  in  the  northern  hemisphere  to  the  right 
(increasingly  as  it  travels  polewards)  so  much 
that  were  it  not  for  the  downward  slope  towards 
the  pole,  it  would  eventually  deviate  towards  the 
equator.  Consequently  the  pressure  to  the  left 
of  its  path,  /".  e.  towards  the  pole,  is  decreased  and 
the  pressure  to  the  right  is  increased.  This 
increases  what  would  otherwise  be  an  insignificant 
slope  to  what  is  actually  observed.  By  the  time 
this  upper  air  has  reached  latitude  30°  which 
divides  the  hemisphere  into  two  equal  areas* 
(though  it  is  only  a  third  of  the  actual  distance 
on  a  meridian)  it  has  descended  to  the  surface 
and  overpowers  any  tendency  towards  contrary 
motion  in  the  air  there,  and  the  entire  atmos- 


*  This  most  important  fact  is  one  of  those  things  which  is 
not  as  a  rule  taught  at  school,  though  it  is  of  immense  signifi- 
cance. 


GENERAL  CIRCULATION  OF  THE  ATMOSPHERE.    77 

phere  tends  to  move  bodily  eastwards  from 
thence  to  the  pole. 

Meanwhile  the  air  near  the  surface  between 
latitude  30°  and  the  equator,  moving  towards 
the  latter,  deviates  towards  the  west  and  heaps 
up  pressure  to  its  right  and  lowers  the  pressure 
to  its  left  in  the  same  way.  Consequently  on  all 
accounts  there  is  a  tendency  on  the  part  of  the 
air  to  heap  up  and  increase  in  pressure  about 
latitude  30°  and  to  be  reduced  in  density  or 
pressure  near  the  poles  and  the  equator.  Also 
since  the  air  reaches  a  terminus  at  the  poles  and 
equator  there  will  be  calms  at  the  surface  at 
both  these  points.  Moreover  since  on  either  side 
of  latitude  30°  or  more  correctly  35°  the  air  moves 
along  the  surface  in  contrary  directions,  there  will 
be  an  absence  of  prevailing  winds  over  this  region. 
These  calms  are  known  to  exist,  and  owing  to 
their  proximity  to  the  tropics  used  to  be  called 
the  calms  of  Cancer  and  Capricorn. 

The  preceding  explanation  may  be  better  real- 
ised if  we  take  a  vertical  section  of  the  atmos- 
phere along  a  meridian  as  it  actually  exists,  and 
draw  sections  of  the  general  planes  of  equal  baro- 
metrical pressure  as  they  exist  by  observation  on 
the  average  at  different  levels  between  the  equator 
and  the  poles  as  in  fig.  (19).  The  poles  are  at  N. 
and  S.  and  the  equator  is  at  Q. 

The  line  A  B  C  D  E  F  represents  a  section  of 
the  general  isobar  of  30  inches  at  sea-level  and 
shews  two  cols  or  hills  at  latitude  30°.  Then  as 
we  ascend  these  cols  gradually  disappear  owing 
to  the  absence  of  the  surface  trades  and  therefore 
of  the  side  pressure  they  create  which  keeps  the 
opposite  pressures  exerted  by  the  winds  on  their 
polar  sides  from  pressing  the  air  into  one  high 


78   THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

pressure  belt  over  the  equator.     As  we  ascend, 
therefore,  these  cols  gradually  coalesce  into  one 


central  hill  from  which  the  air  descends  as  a 
westerly  upper  current  (blowing  partly  from  the 
south  in  the  northern  hemisphere  and  from  the 
north  in  the  southern),  into  the  two  polar  valleys 
on  either  side. 

The  depth  of  the  atmospheric  valleys  of  pres- 
sure below  the  top  level  of  pressure  at  the  surface 
of  the  earth  converted  into  feet  of  air  is  found  to 
be  about  262  feet  at  the  equator,  about  320  feet 
at  the  north  pole,  and  640  feet  at  the  south  pole. 

At  a  height  of  30,000  feet  above  the  surface 
the  north  polar  valley  is  2,800  feet  and  the  south 
polar  valley  3,100  feet  below  the  mean  level. 
The  height  at  which  the  equatorial  valley  dis- 
appears varies  from  8,000  to  12,000  feet.  Above 
this  level  there  is  a  downward  slope  all  the  way 
to  the  arctic  and  antarctic  circles  and  possibly  to 
the  poles  themselves. 


GENERAL  CIRCULATION  OF  THE  ATMOSPHERE.    79 

Viewed  as  a  whole,  the  general  circulation  of 
the  air  according  to  Ferrel,  may  be  considered  as 
consisting  of  two  huge  atmospheric  whirls,  or,  as 
they  are  technically  termed  cyclones,  with  the 
poles  as  centres,  in  which  the  air  rotates  in  each 
hemisphere  in  the  direction  of  axial  rotation. 
On  the  equatorial  side  of  each  of  these  whirls,  a 
belt  occurs  in  which  the  motion  of  the  air  is  con- 
trary to  that  of  axial  rotation.  These  are  the 
trade  wind  belts.  Between  these  two  areas  the 
air  is  heaped  up  into  two  zones  of  high  pressure, 
reaching  their  highest  values  in  the  northern 
hemisphere  about  latitude  35°  and  in  the  south- 
ern about  latitude  30°. 

Since  this  system  was  established  by  Ferrel, 
Dr.  Werner  Siemens,  von  Helmholtz,  Herr  Moller, 
Professor  Oberbeck,  Dr.  Sprung  and  others  have 
developed  the  theory  by  the  aid  of  more  modern 
refined  methods  and  closer  reasoning,  but  their 
conclusions  are  substantially  the  same  as  those 
deduced  by  Ferrel,  and  the  above  sketch  repre- 
sents as  far  as  can  be  attempted  in  a  work  like 
the  present  the  modern  theory  of  the  general  cir- 
culation of  the  atmosphere. 

The  most  noticeable  permanent  modification 
from  the  ideal  condition  of  things  is  afforded  by 
the  exceedingly  low  pressure  round  the  south  pole 
and  the  strong  north-west  winds  which  prevail 
south  of  the  Atlantic,  Indian,  and  Pacific  Oceans, 
and  which  enabled  Australian  clippers  in  the  old 
days  to  make  passages  of  fabulous  rapidity.  This 
is  due  to  the  fact  that  the  southern  hemisphere  is 
chiefly  covered  by  water  which,  by  exerting  less 
friction  on  the  air  than  land,  allows  its  motion* 
to  occur  with  greater  freedom.  In  consequence 
of  this  the  Antarctic  barometric  depression  is 
6 


8o  THE  STORY  Ot  THE  EARTH'S  ATMOSPHERE. 

more  developed  and  more  symmetrical  than  the 
northern.  For  example  the  pressure  on  the  lati- 
tude corresponding  to  our  Antipodes  is  perma- 
nently y^ths  of  an  inch  below  what  we  experience, 
while  the  wind  velocity  is  three  or  four  times  as 
great. 

The  seasonal  changes  and  migrations  as  the 
sun  moves  north  and  south  are  scarcely  notice- 
able in  the  southern  hemisphere  for  the  same  rea- 
son. In  July  the  pressure  over  the  tropical  *  belt 
as  we  may  term  it,  is  slightly  increased,  and  the 
belt  lies  a  little  further  south  than  in  January. 
In  the  northern  hemisphere  on  the  other  hand, 
the  seasonal  changes  are  far  more  conspicuous. 

The  high  pressure  nuclei  which  in  July  lie  on 
the  eastern  sides  of  the  Pacific  and  Atlantic 
oceans  have  by  January  shifted  on  to  the  conti- 
nents of  America  and  Asia,  while  the  low  pres- 
sures, which  in  July  occupied  the  middle  parts  of 
North  America  and  the  centre  of  Africo-Asiatic 
continent,  (the  centre  lying  almost  exactly  over 
the  Persian  Gulf  which  is  the  geographical  land 
centre)  in  January  lie  over  the  North  Pacific  and 
North  Atlantic.  Meanwhile  the  equatorial  low 
pressure  belt,  or  barometric  equator  as  it  may  be 
termed,  which  in  January  is  confined  between  its 
ideal  equatorial  limits,  in  July  runs  up  into  the 
northern  continents  and  in  Africo-Asia  in  particu- 
lar, may  be  said  to  lie  entirely  over  the  land  sur- 
face, where  it  causes  the  Monsoon  as  in  figure 
(20).  The  relation  of  these  extensive  migrations 
to  the  effect  of  seasonal  changes  in  solar  heat  on 


*  The  term  tropical  is  here  used  to  signify  en  or  near  the 
tropics  or  turning  points  and  not  to  the  entire  space  between 
them  as  is  usually  the  case. 


GENERAL  CIRCULATION  OF  THE  ATMOSPHERE.    8  1 


the  air  lying  over  land  and  water  surfaces  is  ob- 
vious. 

The  result  of  these  large  transfers  of  air  and 
air  pressure  north  and  south,  and  between  the 
oceans  and  the  continents,  is  to  cause  what  is 
termed  the  annual  variation  of  the  barometric 
pressure  at  any  single  place  on  the  earth.  This 
annual  variation  reaching  its  extremes  generally 


,/ 


FIG. 


in  January  and  July,  will  be  found  to  be  larg- 
est in  the  centres  of  continents  such  as  Asia 
where  the  barometric  shift  is  greatest  and  least 
on  the  coasts  which  are  the  axes  of  the  annual 


82  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

pressure  sea-saw  between  the  oceans  and  the  con- 
tinents. 

In  England  the  change  is  small,  amounting  to 
about  0.12  inches  between  January  and  July. 

In  India,  it  increases  from  0.26  inches  in  Ben- 
gal to  0.62  inches  in  the  Punjab,  while  over  Siberia 
and  central  Asia  it  reaches  about  i  inch. 

The  mean  pressure  over  the  whole  earth  is 
29.89  inches.  In  the  northern  hemisphere  the 
mean  pressure  for  January  is  29.99,  and  for  July 
29.87.  For  the  southern  hemisphere  the  pressures 
in  the  same  months  are  29.79  and  29.91.  From 
this  it  is  evident  that  there  is  a  much  greater  dif- 
ference between  the  quantity  of  air  over  the  two 
hemispheres  in  the  northern  winter  in  January 
than  in  the  southern  winter  in  July. 

This  difference  in  favour  of  the  northern  hemi- 
sphere really  means  that  owing  to  the  greater 
cooling  and  contraction  over  the  northern  land 
area  in  the  winter  32,000,000  tons  of  air  have 
shifted  over  to  supply  the  defect.  There  is  no 
protective  tariff  placed  upon  this  valuable  import 
from  the  southern  hemisphere. 

A  seasonal  shift  of  the  general  wind  system  of 
the  lower  strata  occurs  like  that  in  the  barometric 
pressures,  as  the  sun  shifts  north  and  south. 

The  shift  of  the  sun  in  latitude  is  47°,  but  the 
wind  systems  only  shift  from  5°  to  8°  on  the  north- 
ern, and  from  3°  to  4°  on  the  southern  side  of  the 
equator.  The  accompanying  diagram  fig.  (21) 
gives  an  idea  of  the  effect  of  the  shift.  The  cen- 
tral belt  (sub-equatorial)  represents  the  district 
which  is  alternately  subject  to  the  doldrums  or 
equatorial  calms,  formerly  the  bane  of  sailors,  and 
the  attendant  bordering  trades  as  they  oscillate 
north  and  south. 


GENERAL  CIRCULATION  OF  THE  ATMOSPHERE.    83 

The  width  of  this  belt  varies  from  350  miles  in 
the  Atlantic  to  200  in  the  Pacific  and  lies  on  the 
north  side  of  the  equator  all  through  the  year,  ow- 
ing to  the  fact  that  the  system  of  circulation  in 
the  southern  hemisphere  and  round  the  South 


FIG.  21. 


pole,  owing  to  smaller  friction,  is  so  much  more 
powerful  than  that  in  the  northern  that  the  pres- 
sure belts  on  that  side  are  all  pushed  northwards. 

The  sub-tropical  belts,  as  the  calms  of  Cancer 
and  Capricorn  are  now  termed,  are  similarly  the 
alternate  arena  of  westerlies,  trades,  and  interven- 
ing calms. 

So  far,  we  have  mostly  considered  the  general 
circulation  with  respect  to  the  motion  of  the  at- 
mosphere, near  the  surface.  The  Upper  current 


84     THE   STORY   OF   THE   EARTH'S   ATMOSPHERE. 

which  blows  all  the  way  from  the  equator  to  the 
poles,  is  usually  termed  the  Anti-trade  in  the 
tropics,  because  it  overlies  and  blows  in  the  con- 
trary direction  to  the  latter  from  the  south-west 
on  the  northern,  and  from  the  north-west  on  the 
southern  side  of  the  wind  equator.  In  the  equa- 
torial zone  its  lowest  limit  is  about  10,000  feet, 
and  as  we  proceed  polewards,  this  limit  descends 
so  that  along  the  range  of  the  Himalaya  in  the 
winter  season,  this  upper  current  descends  to 
within  2000  or  3000  feet  above  the  sea. 

On  the  Peak  of  Teneriffe,  Prof.  Piazzi  Smyth, 
when  he  was  conducting  astronomical  observa- 
tions there  in  1860,  was  able  to  walk  up  through 
the  north-east  trade  wind  and  find  the  south-west 
upper  current  blowing  continuously  at  his  station 
at  Alta  Vista,  10,000  feet  up  the  mountain  side. 

In  like  manner  the  smoke  of  lofty  volcanoes 
such  as  Cotopaxi  18,000  feet,  and  Coseguina  lying 
in  the  trade  wind  zone  have  been  observed  to 
blow  from  the  west,  contrary  to  the  surface  wind. 

Nearer  the  poles  about  latitude  35°  to  40°  the 
lower  edge  of  this  upper  current  touches  the 
earth,  and  its  lower  half  breaks  up  into  what 
Prof.  Helmholtz  terms  vortices.  In  plain  lan- 
guage it  separates  into  irregular  currents  which 
form  the  cyclonic  storms  which  are  so  prevalent 
in  high  latitudes  on  either  side  of  the  equator. 
Meanwhile  the  upper  part  of  the  current,  except 
where  it  is  affected  by  local  disturbances  or 
whirls,  continues  to  move  generally  from  the 
south  or  north-west.  Its  motion  is  motion 
determined  by  observation  of  the  high  clouds 
which  float  at  an  average  elevation,  according  to 
the  most  recent  measurements,  of  about  27,000 
feet. 


GENERAL  CIRCULATION  OF  THE  ATMOSPHERE.    85 

The  general  circulation  of  the  atmosphere  and 
its  seasonal  shifts  is  intimately  bound  up  with  the 
general  distribution  of  the  rainfall  of  the  world, 
and  the  permanent  occurrence  and  seasonal  shift- 
ing of  zones  of  drought  and  rain.  Locally,  rain 
is  due,  especially  in  high  latitudes,  to  the  passage 
of  cyclonic  storms,  but  in  the  equatorial  and 
trade  wind  zones,  the  rainy  season  is  almost  en- 
tirely determined  by  the  shift  of  the  doldrums. 

Without  at  present  going  into  the  question  of 
how  rain  is  produced  in  all  cases,  it  is  easy  to  see 
why  the  central  belt  of  equatorial  calms  is  an  area 
of  constant  cloud  and  rain.  For  the  air  there, 
supplied  with  vapour  by  the  inflowing  trades  on 
either  side,  is  constantly  rising  up  to  higher  and 
colder  levels,  where  it  cannot  contain  so  much 
vapour  as  at  sea-level.  The  surplus  therefore 
condenses  first  into  cloud,  and  then  into  rain- 
drops which  fall  back  to  the  sea-level.  On  the 
equator,  as  at  Singapore,  it  rains  everyday.  As 
the  doldrum  belt  oscillates  north  or  south,  it  may 
give  rise  to  one  rainy  and  dry  season,  or  in  some 
cases  to  two,  thus.  (See  Fig.  22.) 

If  a  place  is  situated  at  a  or  b,  just  within  the 
edge  of  the  rainbelt  at  its  extreme  positions,  it  is 
within  the  belt  during  half  of  the  year  about,  and 
without  it  the  other  half.  Consequently,  it  has  a 
rainy  season  for  six  months,  and  a  dry  season  for 
about  six  months.  This  is  the  case  at  Panama, 
where  it  rains  from  May  to  November,  and  is 
comparatively  dry  during  the  remaining  months. 
Similar  equal  periods  occur  in  Bengal,  the  Nile 
Basin,  and  Northern  Australia.  When  a  place  is 
situated  at  e  or  f,  nearer  the  outer  edge  of  the 
rainbelt  in  its  extreme  position,  the  rainy  season 
is  shorter,  and  the  dry  season  longer.  The  Pun- 


86     THE   STORY   OF   THE   EARTH'S  ATMOSPHERE. 


jab,  Upper  Burmah,  northern  Mexico,  and  north- 
ern Central  Australia  are  regions  which  under ly 
the  rain-belt  for  only  three  months  of  the  year, 
and  if,  as  in  the  year  1896  some  irregularity  oc- 
curs in  the  arrival  of  the  belt,  the  rainy  season 
may  be  so  short  as  to  cause  drought  and  famine. 

Extreme  Northern'     Extreme  Southern, 
position  ofrainbelt      position  of  -ainbelt 


FIG.  22. 

Places  such  as  g  and  h,  lying  outside  the  influ- 
ence of  the  equatorial  rainbelt  altogether,  would 
be  rainless  except  for  the  extension  equatorwards 
of  the  system  of  polar  winds,  which  sometimes 
descends  as  far  south  as  latitude  35°  in  the  win- 
ter. Between  latitude  35°  and  latitude  20°  N.  and 
S.,  except  where  as  in  India,  monsoons  blow  from" 
the  equator,  or  along  coast  lines,  no  regular  rain- 
fall belt  arrives,  and  the  dry  desert  zones  of  the 
earth  tend  to  form. 

The  dry  regions  of  California,  Arizona,  and 
Colorado  in  North  America,  the  great  Sahara  and 


GENERAL  CIRCULATION  OF  THE  ATMOSPHERE.    87 

Nubian  deserts  of  North  Africa,  and  the  Arabian 
and  Persian  dry  areas  all  occur  between  these 
limits  in  the  northern  hemisphere. 

In  the  Southern  hemisphere  within  the  same 
parallels  are  the  dry  regions  of  the  Argentine  and 
Eastern  Patagonia,  a  large  dry  region  in  South 
Africa  and  one  comprising  the  whole  interior  of 
Australia.  These  areas  coincide  with  the  belts 
of  high  atmospheric  pressure  and  may  be  said  to 
suffer  from  permanent  fine  weather. 

Places  at  c  between  the  two  positions  of  the 
equatorial  rainbelt,  experience  two  short  dry  sea- 
sons alternated  by  two  short  seasons  of  rainfall. 
Such  are  Ceylon  and  southern  India,  Colombia  in 
South  America,  parts  of  the  Nile  basin  and  Java. 

In  the  latitudes  between  35  degrees  and  the 
poles,  the  seasonal  rains  are  entirely  regulated 
by  the  seasonal  shifts  in  the  polar  system  of  gen- 
eral winds. 

Here  again,  owing  to  the  shift  in  latitude  of 
a  principal  single  rain  belt  corresponding  to  the 
seasonal  shift  of  the  sun,  some  places  in  the  mid- 
dle of  the  area  between  its  extreme  limits  un- 
dergo two  rainy  and  two  dry  periods.  In  South 
Europe,  for  example,  the  rain  falls  mostly  in  the 
winter,  because  the  system  of  winds  circling  round 
the  pole  reaches  its  furthest  extension  towards 
the  equator  at  that  season.  In  middle  Europe, 
the  rains  fall  chiefly  in  spring  and  autumn  as  the 
belt  moves  north  and  returns,  and  in  Northern 
Europe  they  fall  chiefly  in  the  summer. 

The  general  circulation  of  the  atmosphere  not 
only  determines  the  prevalent  direction  of  the 
winds  on  the  surface  and  in  the  upper  regions, 
but  also  exercises  a  very  large  influence  upon 
their  average  velocity. 


88     THE   STORY  OF  THE   EARTH'S  ATMOSPHERE. 

The  practical  importance  of  possessing  a 
knowledge  of  the  general  velocity  of  motion  of 
the  air  above  the  earth's  surface  is  evident  when 
we  touch  upon  the  subject  of  ballooning  or  the 
coming  flight  of  man. 

When  man  is  able  to  circumnavigate  the  ocean 
of  air  with  the  same  ease  that  he  sails  across  the 
ocean  of  water  he  will  require  to  possess  as  accu- 
rate a  knowledge  of  atmography  (to  invent  a  title) 
as  he  does  at  present  of  hydrography.  We  have 
at  present  a  hydrographer  to  the  admiralty,  and 
we  shall  then  require  the  services  of  one  who  will 
tell  us  all  about  the  movements  and  conditions  of 
the  air,  not  only  at  sea-level,  but  in  the  upper 
regions  whither  it  will  be  necessary  to  ascend  in 
order  to  cross  mountain  chains. 

From  the  theory  of  circulation  as  developed 
by  Ferrel  and  Oberbeck,  it  appears  that  the 
surface  wind  ought  to  reach  its  greatest  average 
velocity  about  latitude  50°,  and  diminish  thence 
both  towards  the  poles  on  the  one  side  and 
towards  the  equator  on  the  other.  As  a  matter 
of  observation  this  appears  to  be  the  case. 

Taking  an  average  of  the  winds  throughout 
the  year,  the  late  Prof.  Loomis  found  the  follow- 
ing average  values  for  the  wind  in  typical  lati- 
tudes : — 

Mean  velocity  of  wind 
in  miles  per  hour. 

United  States 9.5 

Europe 10.3 

Southern  Asia 6.5 

West  Indies 6.2 

In  England  the  average  surface  wind  is  nearer 
12  miles  an  hour.  Like  other  elements  the  sur- 
face wind  varies  with  distance  from  the  sea,  time 
of  year,  and  time  of  day. 


GENERAL  CIRCULATION  OF  THE  ATMOSPHERE.    89 

The  movements  of  the  air  are  much  affected 
by  the  nature  of  the  surface  over  which  it 
passes. 

It  moves  faster  over  water  than  over  land, 
and  faster  over  flat  bare  land  than  where  it  is 
hilly  or  covered  with  forest.  In  the  interior  of 
continents  it  is  much  more  sluggish  than  near  the 
coasts  or  out  at  sea. 

Thus  in  India,  the  wind  velocity  diminishes 
as  we  leave  the  coast  in  the  following  manner: — 

Towns  Velocity  of  wind  in 

miles  per  diem. 

~  )  Bombay 408 

On  coast  Uurrachee 497 


£Sf«— * 

Again,  while  the  velocity  over  Europe  is  10 
miles  an  hour,  it  is  as  much  as  29  miles  an  hour 
over  the  North  Atlantic. 

Everyone  is  aware  of  the  great  amount  of 
wind  experienced  even  in  summer  when  crossing 
the  Channel  as  compared  with  that  felt  on  shore. 

A  similar  difference  of  velocity  is  observed  as 
we  ascend  from  the  earth's  surface. 

This  is  partly  due  to  the  decrease  of  friction 
and  partly  to  the  increased  slope  down  which  the 
upper  air  raised  by  the  equatorial  heat  tends  to 
flow  towards  the  poles. 

Near  the  surface  and  for  the  first  50  or  100 
feet  the  increased  velocity  with  height  is  entirely 
due  to  the  diminished  friction  encountered  by  the 
air  against  the  roughness  of  the  surface,  trees, 


90  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

houses,  and  other  obstacles.  After  that  the 
retardation  of  the  lower  layers  is  communicated 
to  the  upper  ones  in  a  gradually  decreasing  scale 
by  means  of  the  friction  of  the  air  molecules 
against  each  other,  somewhat  as  a  spoon  passed 
through  treacle  or  honey  drags  some  of  the  sur- 
rounding mass  along  besides  what  it  pushes 
directly  in  front  of  it. 

This  property  of  the  particles  of  a  gas  is 
termed  viscosity,  owing  to  its  similarity  to  the 
visible  behaviour  of  what  is  termed  a  viscous 
liquid  like  melted  glass. 

Such  resistance  which  neighbouring  portions 
of  gas  offer  to  one  another's  motion  is  due,  on 
the  Kinetic  theory  of  gases,  to  the  collisions 
which  are  constantly  taking  place  between  the 
rapidly  oscillating  molecules. 

A  good  parallel  is  offered  by  a  crowd  of  per- 
sons all  moving  along  a  road  in  the  same  general 
direction  towards  some  common  point  of  interest. 
If  everyone  moved  at  the  same  pace  in  parallel 
lines,  the  speed  of  the  crowd  would  be  the  same 
as  that  of  any  individual  person,  but  owing  to 
the  fact  that  some  persons  cannot  walk  so  quickly 
as  others,  some  stop  to  look  at  the  shop  windows, 
others  walk  crookedly  and  jostle  their  neighbours, 
while  some  walk  back  against  the  crowd  because 
they  have  left  something  behind  them,  the  average 
speed  of  the  crowd  is  sensibly  reduced  below  that 
of  the  quickest  walkers,  though  it  still  remains 
above  that  of  the  slowest. 

In  like  manner  if  two  streams  of  persons,  one 
moving  faster  than  the  other,  join  together  and 
personal  interchanges  take  place  between  them, 
the  persons  who  walk  across  from  the  slower 
stream  into  the  quicker  one  tend  to  retard  its 


GENERAL  CIRCULATION  OF  THE  ATMOSPHERE-   91 

average  pace.  Those  on  the  contrary  who  move 
across  from  the  quicker  stream  into  the  slower 
one  tend  to  accelerate  its  average  pace. 

In  a  similar  manner,  adjacent  strata  of  the 
atmosphere  tend  to  equalise  each  other's  speed 
on  a  small  scale  by  interchange  of  molecules, 
and  where  large  masses  intermingle,  as  in  the 
general  circulation,  by  interchange  of  masses, 
moving  with  different  average  speeds. 

It  is  by  this  internal  friction  between  inter- 
mingling air  masses  in  addition  to  that  experi- 
enced by  the  friction  of  the  lowest  layer  against 
the  earth,  which  is  gradually  communicated  to 
those  above,  that  the  atmosphere  does  not  assume 
unheard-of  velocities  and  storms  are  not  more 
violent  than  they  are.  The  latter  alone  would 
not  be  enough. 

Helmholtz,  for  example,  has  calculated  that  if 
the  atmosphere  generally  started  moving  over  the 
earth  with  a  certain  average  velocity,  say  of  20 
miles  an  hour,  it  would  take  no  less  than  42,747 
years  to  reduce  this  to  10  miles  an  hour  by  the 
action  of  friction  with  the  ground. 

The  first  experiments  to  find  the  increase  of 
velocity  of  the  air  with  the  height  above  the 
ground  were  undertaken  by  Mr.  T.  Stevenson 
of  Edinburgh,  who  attached  anemometers  of  the 
Robinson  pattern  at  different  heights  on  a  5o-foot 
pole.  Here  the  retarding  effect  of  the  ground 
was  found  to  rapidly  diminish  in  the  first  15  feet. 
Above  this  the  rate  decreased.  For  building  and 
engineering  purposes  it  is  best  to  make  experi- 
ments in  the  locality  and  trust  to  no  formula, 
since  the  rule  alters  rapidly  up  to  the  first  100  feet. 

Beyond  this  and  up  to  1800  feet  experiments 
conducted  by  the  author  in  1883-5,  under  a  grant 


92      THE   STORY   OF  THE   EARTH'S   ATMOSPHERE. 

from  the  Royal  Society,  resulted  in  establishing 
the  fact  that  the  average  velocity  at  1600  feet  is 
just  double  the  velocity  at  100  feet.  Above  the 
former  height  the  rate  appears  to  increase,  if  we 
are  to  judge  from  the  observations  of  the  clouds 
made  at  Blue  Hill  Observatory,  near  Boston, 
U.  S.  A.  Mr.  Clayton's  recent  observations  there 
of  the  movements  of  the  different  cloud  strata 
reduced  to  English  measure  give  the  following 
results  throughout  the  year: — 


Cloud  level. 
Stratus       , 

Height         A 
in  feet.            r 
,  i  676 

verage  speed  in 
niles  per  hour. 

IQ 

5,326 

24 

12  724 

34 

21  888 

Cirrus  .  .  . 

.  .    20.117 

78 

The  rule  in  this  case  may  be  simply  remem- 
bered thus.  For  every  1000  feet  of  ascent  add 
on  about  2  miles  an  hour  to  the  velocity  of  motion. 

In  winter  the  speeds  are  twice  as  large  at  the 
upper  levels  as  in  summer.  For  the  winter  half 
year  the  speed  of  the  cirrus  is  as  much  as  96 
miles  an  hour,  considerably  faster  than  our  ex- 
press trains  travel  at  present. 

In  Europe  the  velocities  appear  to  increase 
less  rapidly,  but  are  still  large  when  compared 
with  those  at  the  surface. 

An  average  of  closely  concordant  results, 
obtained  by  Dr.  Vettin  of  Berlin  and  Hagstrom 
and  Dr.  Ekholm  of  Upsala  in  Sweden,  make  the 
velocities  at  4300  and  22,000  feet  about  19  and 
38  miles  per  hour  respectively.  Here  the  rule 
gives  about  i  mile  per  hour  for  each  1000  feet  of 
elevation,  which  is  probably  nearer  the  mark  for 
Europe  generally. 


GENERAL  CIRCULATION  OF  THE  ATMOSPHERE.    93 

This  great  increase  of  velocity  of  the  average 
motion  of  the  aerial  ocean  as  we  rise  above  the 
surface  is  scarcely  realised  by  us  tiny  mortals 
who  dwell  mostly  at  its  base. 

The  loftiest  building  is  scarce  1000  feet  above 
the  ground,  while  the  loftiest  inhabited  place  is 
but  half-way  through  the  mass  and  probably  a 
twentieth  of  the  actual  height  of  the  atmosphere. 
The  great  velocity  often  attained  by  balloons  is 
thus  readily  explained. 

At  the  same  time  it  is  equally  plain  that  no 
navigable  balloon  will  ever  be  able  to  stem  the 
currents  above  5000  feet,  while  flying  machines 
would  do  well  to  travel  with  the  wind  above  this 
elevation.  In  fact  they  may  eventually  utilise 
these  rapid  currents  much  in  the  same  way  as  the 
Australian  clippers  formerly  utilised  the  brave 
north-west  wind  which  blows  so  powerfully  round 
the  watery  expanse  of  the  southern  ocean. 

One  very  curious  result  of  this  great  motion  at 
high  altitudes  has  been  recently  pointed  out  by 
Herr  Moller,  a  German  engineer.  The  energy  of 
the  motion  of  the  air  or  the  power  it  possesses  of 
performing  work  is  proportional  to  its  speed  and 
mass.  Moller  has  thus  calculated  that  the  energy 
of  the  upper  half  of  the  atmosphere — /.  e.,  the  half 
above  16,000  feet,  is  no  less  than  six  times  the 
energy  possessed  by  the  lower  half.  Of  this  in- 
conceivably enormous  store  of  energy  we  at 
present  utilise  a  minute  proportion  in  sailing 
ships  and  driving  windmills.  The  rest  is  com- 
pletely wasted. 


94  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

CHAPTER   VI. 

THE    LAWS    WHICH    RULE    THE    ATMOSPHERE. 

THE  story  of  the  earth  is  for  the  most  part  a 
chapter  of  ancient  history.  The  story  of  the  at- 
mosphere is  a  tale  of  to-day,  and  even  of  to-mor- 
row. When  we  have  opened  up  the  earth's  crust 
to  view  we  can  trace  the  operation  of  past  changes 
in  the  physical  and  chemical  nature  and  position 
of  the  different  rocks.  All  the  atmospheric  mo- 
tions and  changes,  on  the  other  hand,  are  going 
on  before  our  eyes.  Every  action,  moreover,  is 
subject  to  the  reign  of  law.  The  chaos  which  at 
first  sight  appears  to  surround  the  infinite  com- 
plexity of  atmospheric  phenomena  is  reduced  to 
harmony  and  order  in  proportion  as  we  learn  the 
true  laws  which  operate  in  the  grand  laboratory 
of  Nature.  We  have  been  a  long  time  learning 
our  lesson,  and  we  are  only  now  beginning  to  rise 
from  superstitions  and  guesses  to  those  intellectual 
"Delectable  mountains"  whence  "we  may,  even 
though  it  be  "through  a  glass  darkly,"  snatch  a 
glimpse  of  the  true  character  of  the  mysteries  of 
our  wonderful  atmosphere. 

The  earth  is  a  symbol  of  rest,  stability,  and 
permanence.  The  atmosphere,  on  the  other 
hand,  is  in  ceaseless,  motion  and  constant  ac- 
tivity, under  the  influence  of  the  heat  of  the  sun, 
the  cold  of  space,  the  rotation  of  the  earth,  and 
the  changes  of  the  seasons,  as  it  moves  in  its 
orbit  round  the  sun.  Physicists  working  in  their 
laboratories  have  discovered  that  certain  laws 
are  obeyed  by  air  in  common  with  other  gases, 
and  it  is  only  when  we  know  these  laws  that  we 


THE   LAWS   WHICH    RULE   THE   ATMOSPHERE.    95 

can  interpret  the  phenomena  which  are  daily  and 
hourly  observed  in  the  sky  and  air  around  us. 
One  of  the  first  laws  relating  to  the  atmosphere 
was  discovered  by  Dr.  Boyle,  the  celebrated 
Glasgow  chemist,  and  Marriotte  of  France,  and 
is  usually  called  Boyle  and  Marriotte's  law. 
This  law  states  that  if  a  volume  of  gas  (which  is 
elastic  and  compressible),  confined  within  certain 
limits,  such  as  an  elastic  bag,  is  subjected  to 
compression,  its  pressure  increases  in  the  same 
proportion  as  its  volume  decreases.  Thus,  if  6 
cubic  feet  of  air  at  the  ordinary  atmospheric 
pressure  were  squeezed  together  until  they  occu- 
pied only  3  feet,  the  pressure  or  resistance  of  the 
air  would  rise  to  that  of  two  atmospheres;  and 
if  a  mercury  barometer,  at  first  marking  30 
inches,  were  placed  within  the  containing  vessel, 
the  column  of  mercury  would  at  the  end  of  the 
experiment  rise  to  a  height  of  60  inches. 

Human  beings,  when  subjected  to  moral 
pressure,  frequently  exhibit  similar  characteris- 
tics, though  their  resistance  cannot  be  measured  on 
a  moral  barometer.  In  the  free  atmosphere  such 
an  ideal  case  seldom  occurs,  since  the  air  gener- 
ally finds  some  escape  from  complete  compression 
by  expansion  or  motion  in  various  directions. 

One  immediate  result  of  this  law  is  the  great 
density  of  the  lower  strata  of  the  atmosphere, 
due  to  the  compression  to  which  they  are  sub- 
jected by  the  weight  of  the  overlying  layers.  In 
like  manner  the  rarity  of  the  upper  air  is  due  to 
its  smaller  compression.  Also,  when  any  mass  of 
air  is  forced  upwards,  it  comes  under  gradually  de- 
creasing pressure,  and  consequently  by  Boyle's  law 
it  expands.  Conversely,  if  it  is  forced  downwards, 
it  contracts  owing  to  the  increasing  pressure. 


96  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

It  is  important  to  notice  the  distinction  be- 
tween cases  where  the  air  is  forced  upwards  and 
where  it  ascends  by  reason  of  an  expansion  al- 
ready effected  before  it  starts,  by  the  action  of 
heat.  In  the  former  case  it  stops  when  the 
forcing  agency  stops.  In  the  latter  it  rises  to 
the  level  where  the  air  all  round  is  equally  ex- 
panded, and  therefore  of  the  same  density. 

The  former  case  occurs  in  Nature,  where  a 
wind  blows  athwart  an  abrupt  chain  of  moun- 
tains, and  forces  the  air  up  their  sides.  The  lat- 
ter occurs  wherever  air  is  locally  heated  above  or 
cooled  below  that  surrounding  it. 

When,  instead  of  being  compressed,  the  air  is 
heated  within  a  confined  vessel,  its  pressure  in- 
creases in  direct  proportion  to  its  rise  in  tem- 
perature (when  reckoned  from  an  absolute  zero 
461°  below  zero  Fahr.).  If  when  heated  it  is 
allowed  to  expand  freely  (that  is  to  say,  still 
confined  by  ordinary  atmospheric  pressure)  it  ex- 
pands or  increases  in  volume  in  like  proportion. 
This  is  called  the  law  of  Charles  or  Gay-Lussac. 

A  third  law  which  is  really  the  converse  of 
this  may,  for  convenience,  be  termed  Poisson's 
law,  and  asserts  that,  if  air  is  suddenly  compressed 
it  rises  proportionately  in  temperature,  and  if 
suddenly  allowed  to  expand,  it  falls  in  tempera- 
ture. The  suddenness  is  only  necessary  in  order 
that  the  heat  engendered  may  not  have  time  to 
escape  before  it  can  be  detected.  These  three 
laws,  in  combination  with  a  few  special  charac- 
teristics displayed  by  water  vapour,  explain  all 
the  varieties  of  atmospheric  phenomena  primarily 
due  to  the  action  of  heat  and  cold,  such  as  wind, 
storms,  clouds,  rain. 

These  laws  are  really  only  variations  of  one 


THE  LAWS  WHICH  RULE  THE  ATMOSPHERE.   97 

grand  principle  which  applies  to  everything  in 
the  Universe — viz.,  what  is  termed  the  "  conser- 
vation of  energy."  Thus,  to  take  a  single  exam- 
ple, when  a  bullet  hits  a  target  it  gets  quite  hot, 


FIG.  23. — "AFTER  THE   STORM." 

From  the  Croix  de  fer  Switzerland,  7000  feet  above  sea-level. 

and  the  heat  that  is  thus  generated  is  the  exact 
equivalent  of  the  motion  that  is  lost. 

Heat,  as  we  have  learnt,  is  "  a  mode  of  mo- 
tion." It  is,  in  fact,  a  motion  of  the  small  atoms 
or  molecules  that  make  up  a  body  instead  of  a 
motion  of  the  body  itself. 

According  to  the  modern  theory  of  gases, 
which  applies  equally  to  a  mixture  of  gases  such 
as  air,  the  tiny  atoms  or  molecules  which  com- 


98  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

pose  them  are  in  a  state  of  constant  motion  back- 
wards and  forwards,  kicking,  as  it  were,  against 
each  other,  and  against  anything  that  obstructs 
their  freedom  to  move.  The  pressure  exerted  by 
a  gas  on  its  neighbourhood,  where  this  is  a  solid 
liquid  or  gas,  is  measured  by  the  number  of  kicks  or 
impacts  which  its  atoms  perform  in  a  given  time. 

If  the  gas  contained  within  a  bag  (say)  is 
compressed,  the  paths  of  the  atoms  are  shortened, 
and,  in  consequence,  the  number  of  impacts  with 
each  other  and  against  the  sides  of  the  bag  is 
increased — i.  e,,  the  pressure  increases.  That  is 
Boyle's  law. 

In  like  manner,  if,  without  compression,  the 
temperature  of  the  gas  is  raised,  the  speed  of 
the  movements  is  increased.  (Molecular  speed  is 
heat.)  Therefore,  the  effect  in  this  case  is  just  the 
same  as  when  the  gas  was  compressed  and  the 
paths  were  shortened — viz.,  an  increased  number 
of  impacts  or  kicks,  and  therefore  increased  pres- 
sure. That  is  Charles's  law. 

When  the  gas  is  suddenly  compressed,  the 
atoms  have  been  pushed  towards  each  other,  and 
their  speed  has  therefore  been  increased  by  this 
push.  Consequently,  a  rise  of  temperature  takes 
place.  This  is  Poisson's  law. 

In  all  these  cases,  no  energy  is  lost  or  created. 
It  is  simply  a  transformation  of  motion  into  dif- 
ferent "  modes,"  from  which  it  can  be  retrans- 
formed  without  sensible  loss,  though  it  is  easier 
to  transform  motion  into  heat  than  heat  into  mo- 
tion. Dr.  Joule,  of  Manchester,  was  the  first  to 
determine  the  exact  equivalence  between  what  we 
term  motion  and  heat.  His  corrected  law  may  be 
stated  thus : 

When  a  pound  weight  falling  through  783  feet  is 


THE   LAWS   WHICH    RULE  THE  ATMOSPHERE.    99 

arrested,  as  much  heat  will  be  developed  as  will  raise 
one  pound  of  water  one  degree  above  absolute  zero* 

The  converse  is  equally  true.  Here  we  see 
the  true  cause  of  the  marvellous  manifestations 
of  energy  in  the  movements  of  the  atmosphere, 
the  devastating  hurricanes  which  overturn  build- 
ings and  destroy  ships,  and  the  terrible  tornadoes 
of  America  which  have  been  known  to  carry  solid 
objects  like  wooden  church  spires  a  distance  of 
15  miles  and  kill  hundreds  of  people  in  the  space 
of  a  few  minutes.  They  are  all  due  to  the  heat 
of  the  sun,  which  is  converted  into  motion  on  the 
above  scale. 

Charles's  law,  by  which  air  expands  by  heat, 
and  Poisson's,  by  which  it  cools  by  expansion 
under  diminished  pressure,  have  a  very  important 
bearing  on  the  formation  of  clouds,  rain,  thunder- 
storms, tornadoes  and  tropical  cyclones. 

When  a  mass  of  dry  air,  or  air  only  containing  a 
small  proportion  of  vapour,  rises,  it  cools  at  the  rate 
of  i°  F.  for  every  183  feet  it  ascends.  This  would 
be  about  1.6°  in  300  feet,  or  about  5.2  in  1000  feet. 

The  rate,  however,  at  which  the  air  is  found 
by  observation  to  remain  cooler  as  we  ascend  in 
the  atmosphere  is  much  slower  than  this.  Con- 
sequently, if  dry  air  is  warmed  up  until  it  expands 
and  gets  lighter  than  the  surrounding  air,  it  can- 
not rise  very  far  before  it  is  cooled  by  ascent,  down 
to  the  temperature  and  density  of  the  air  around 
it  at  the  same  level,  when  it  is  bound  to  stop. 

When,  however,  air  contains  as  much  vapour 
as  it  can  hold  invisibly,  or  is  said  to  be  saturated, 
the  case  is  different.  As  it  ascends  and  cools,  the 
vapour  condenses  into  cloud  and  finally  falls  out 

1  (*.  e.,  461°  below  zero  Fahr.) 


100  THE   STORY   OF  THE   EARTH'S  ATMOSPHERE. 

as  rain  and  at  the  same  time  gives  out  the  heat 
which  is  absorbed  in  the  act  of  conversion  from 
water  into  vapour.  This  heat,  which  is  latent  so 
long  as  it  is  vapour,  becomes  patent  when  it  con- 
denses and  retards  the  cooling  of  such  a  mass  of 
air  so  much  that  under  ordinary  conditions  of 
temperature andpressure  in  these  latitudes(barom- 
eter  30  inches,  thermometer  62°),  it  only  cools 
-fths  of  a  degree  Fahr.  in  ascending  300  feet.  An 
ascending  current  of  air  saturated  with  vapour 
once  started,  therefore,  could  go  on  ascending 
until  nearly  all  the  vapour  had  fallen  out  of  it,  or 
until  it  had  risen  into  very  lofty  and  cold  regions 
of  the  atmosphere. 

Air  saturated  with  vapour  is  thus  essential  to 
the  formation  of  clouds,  especially  cumulus  clouds, 
like  that  in  our  frontispiece,  and  of  upward  cur- 
rents generally,  which  are  the  chief  cause  of  local 
storms.  Diffusion  by  which  one  gas  tends  to 
work  its  way  through  another  plays  an  impor- 
tant role  in  atmospheric  economy.  Were  it  not 
for  diffusion  the  heavier  gases  would  all  lie  near 
the  surface  and  the  lighter  near  the  top,  and  we 
should  all  be  poisoned  off  by  even  the  small 
amount  of  carbonic  acid  there  is. 

Some  years  ago  the  great  chemist  Dalton 
founded  the  law  of  gaseous  pressure,  and  deduced 
that  of  diffusion  from  it,  but  it  has  since  been 
found  that  though  his  conclusions  were  fairly  cor- 
rect, they  are  not  due  to  the  causes  he  alleged. 
The  rule  is  that  the  lighter  the  gas  the  more  rap- 
idly it  tends  to  wander  through  its  neighbours, 
and  the  tendency  is  for  each  gas  to  behave  as 
though  its  neighbours  were  not  in  existence. 
Thus,  the  tendency  is  for  the  component  gases  of 
the  atmosphere  to  take  up  positions  in  which  they 


THE   LAWS   WHICH  RULE  .THE   ATMOSPHERE.   IOI 

would  exist  as  separate  atmospheres  up  to  the 
limits  in  miles  indicated  opposite  each  in  the  ad- 
joining diagram. 

This  ideal  final  arrangement  never  takes  place 


80 

70 

•60 

62 

-Hydrogen. 

50 

4-0 

37 

-Water  Vapour* 

30 

32 
31 

.  tfitroyen/. 

20 

10 
c\ 

9 

-  Carbonic  acid 

FIG.  24. — Diffusive  limits  of  the  component  gases  of  the 
atmosphere. 

owing  to  the  constant  motion,  but  since  all  the 
gases  diffuse  upwards  as  well  as  downwards  it  is 
quite  possible  that  some  of  the  hydrogen  and 
lighter  gases  have  diffused  upwards  until  they  have 
got  beyond  the  power  of  recall  by  gravitation. 

Long  before  the  water  vapour  has  reached  37 
miles,  a  great  deal  is  lost  by  being  condensed 
into  rain.  At  a  height  of  9  miles  above  the  sur- 
face, for  example,  the  actual  amount  of  vapour 


102  THE   STORY   OF   THE   EARTH'S   ATMOSPHERE. 

present  is  only  g^th  of  what  would  exist  if  it 
were  incondensibie. 

The  laws  which  determine  the  passage  of  the 
sun's  heat  and  light  through  the  atmosphere 
present  problems  which  are  even  yet  only  par- 
tially solved,  partly  because  light  and  heat  are 
made  up  of  a  variety  of  wave  movements  in  that 
wonderful  medium  which  pervades  all  space 
termed  aether,  and  partly  because  the  conditions 
in  the  atmosphere  can  never  be  exactly  imitated 
in  the  laboratory. 

However,  this  much  is  known.  Ordinary 
white  light  from  the  sun  when  passed  through  a 
prism  is  found  to  be  made  up  of  a  variety  of 
coloured  rays  ranging  from  violet  to  red.  The 
visible  violet  rays  are  made  up  of  the  smallest 
waves,  about  .00001  in.  in  length,  and  the  visible 
red  rays  of  waves  about  .00003  i°-  Beyond 
these  visible  rays  are  invisible  ones  which  affect 
the  atmosphere  and  earth,  and  whose  existence 
can  be  proved  by  photography.  The  rays  to- 
wards the  red  end  produce  more  heat  than  light, 
and  those  towards  the  violet  end  more  light  than 
heat.  The  general  action  of  the  atmosphere  on 
these  rays  from  the  sun  is  twofold.  In  passing 
through  it  they  are  partly  absorbed  and  partly 
scattered.  The  blue  and  violet  rays  are  most 
affected,  and  the  red  least.  In  fact,  as  Prof. 
Langley  has  pointed  out,  so  much  of  the  blue 
rays  are  filtered  out  by  the  atmosphere  that  if  we 
could  see  the  sun  as  he  appears  at  the  outside 
of  our  atmosphere  he  would  be  blue  instead  of 
white. 

The  absorption  particularly  of  the  heat  rays 
has  been  shown  by  the  late  Prof.  Tyndall  to 
be  mainly  effected  by  the  water-vapour  present 


THE   LAWS  WHICH   RULE   THE   ATMOSPHERE.    103 

in  the  atmosphere,  while  the  scattering  is  effected 
by  the  fine  particles  of  dust  and  frozen  vapour. 

The  red  colours  at  sunset  are  due  to  the  fact 
that  the  sun's  rays,  before  they  reach  our  eyes 
in  this  position,  pass  through  a  thickness  of  at- 
mosphere about  900  miles  instead  of  50  miles  when 
overhead.  The  blue  rays,  in  consequence,  are 
nearly  all  filtered  out,  and  nothing  is  left  but  the 
longer  waves  at  the  red  end  of  the  prismatic  spec- 
trum. The  red  colour  of  the  sun  when  seen 
through  a  fog  is  due  to  a  like  excess  of  absorption 
and  scattering  of  blue  rays  by  the  particles  of  fog. 

Similarly,  the  blue  colour  of  the  sky  when  the 
sun  is  high  up  in  the  heavens  is  explained  by 
Lord  Rayleigh  to  be  due  to  these  missing  blue 
rays.  Scattered  mostly  at  right  angles  to  the  di- 
rection of  the  sun's  rays,  these  spurned  rays  reach 
us  from  all  parts  of  the  sky,  and  shew  up  blue 
against  what  would,  in  their  absence,  be  the  black 
background  of  space. 

When  the  luminous  rays  from  the  sun  pass 
through  the  air,  they  heat  the  dry  part  of  it  very 
little.  The  absorption  is  mainly  effected  by  the 
small  but  valuable  vapour  constituent,  which 
Prof.  Hill  estimates  as  being  764  times  as  effect- 
ive as  dry  air.  The  remaining  rays  on  reaching 
the  earth  are  changed  into  invisible  heat  rays, 
which  possess  the  peculiar  property  of  being  un- 
able to  repenetrate  the  atmosphere.  Trapped 
thus  like  lobsters  in  a  basket,  they  expend  their 
energy,  first  upon  the  earth's  surface  and  then 
upon  the  adjacent  air.  The  atmosphere,  in  fact, 
acts  like  the  glass  in  a  greenhouse,  which  lets  the 
luminous  rays  in  and  prevents  their  escape  when 
they  have  become  converted  into  dark  heat. 

The  upper  parts  of  the  atmosphere  would  thus 


104   THE   STORY  OF   THE  EARTH'S   ATMOSPHERE. 

remain  colder  than  they  are,  were  it  not  for  the 
conveyance  (convection  is  the  technical  word)  of 
the  heat  from  the  lower  strata  to  the  upper  re- 
gions— in  other  words,  from  the  ground  floor  to 
the  attics.  This  convection  plays  such  an  impor- 
tant part  in  the  atmospheric  economy,  and  in  the 
formation  of  clouds,  rain,  and  storms  of  every 
kind,  that  it  demands  a  brief  consideration. 

We  have  seen  that  in  general,  ascending  air 
cools  at  the  rate  of  i°  F.  for  every  183  feet.  Con- 
sequently, so  long  as  the  rate  of  decrease  of  tem- 
perature with  the  height  is  equal  to  or  slower  than 
this,  the  atmosphere  tends  to  remain  in  vertical 
equilibrium — that  is  to  say,  vertical  motions  will 
not  arise  spontaneously.  The  only  interchange 
under  such  circumstances  would  be  due  to  expan- 
sion and  overflow,  such  as  that  described  in 
Chap.  IV.,  and  which  gives  rise  to  the  general  cir- 
culation between  the  equator  and  the  poles  des- 
cribed in  Chap.  V.  When,  however,  the  atmos- 
phere near  the  surface  is  not  only  heated  by 
radiation  of  the  changed  solar  rays  by  the  earth, 
but  is  surcharged  with  vapour,  it  cools  even  at 
62°  F.,  our  summer  temperature  in  England,  only 
i°  F.  in  every  400  feet  of  ascent,  while  at  92°  F., 
as  often  occurs  within  the  tropics,  it  cools  only 
i°  F.  in  500  feet.  An  upward  movement  once 
started,  therefore,  is  able  to  continue,  since  the 
air  is  always  warmer,  and  therefore  lighter  than 
that  which  it  reaches  above.  Similar  downward 
motions  of  the  cool  air  above  take  place  until  a 
large  proportion  of  the  heat  received  near  the 
surface  is  carried  up  aloft.  The  process  is  pre- 
cisely analogous  to  that  by  which  the  hot  water 
from  the  kitchen  boiler  is  conveyed  through  the 
pipes  to  the  cisterns  at  the  top  of  a  house. 


THE   LAWS  WHICH   RULE   THE   ATMOSPHERE.    105 

These  upward  convection  currents  carry  the 
life-giving  heat  to  the  cold  regions  above  us,  just 
as  the  arterial  blood  conveys  warmth  to  our  ex- 
tremities, and  are  quite  as  necessary  to  the  life  of 
the  atmosphere  as  the  circulation  of  blood  heat  is 
to  that  of  the  animal.  Were  it  not  for  this  safe 
conveyance,  moreover,  of  the  suplus  heat  away 
from  our  midst,  there  would  often  be  a  dangerous 
accumulation  which  would  render  life  more  insup- 
portable than  it  is,  especially  in  the  tropics.  When 
the  existing  rate  of  decrease  becomes  anything 
like  i°  F.  in  100  feet,  or  greater,  rapid  convection 
sets  in,  even  if  the  air  is  comparatively  dry,  and 
clouds  are  formed  with  rain,  and  often  lightning. 
If  the  air  over  a  large  district  is  affected,  as  some- 
times occurs  in  the  tropics,  a  cyclone  is  formed 
by  the  inrush  of  surrounding  air,  or,  if  the  action 
is  very  intense  and  quite  local,  a  tornado  or  whirl- 
wind may  result.  This  may  be  termed  convection 
run  riot. 

Prof.  Abbe",  of  the  U.  S.  Weather  Bureau,  con- 
siders the  limit  of  convection  currents  to  be 
about  30,000  feet,  or  about  the  height  of  Mount 
Everest.  Above  this  the  temperature  diminishes 
very  rapidly,  as  indeed  we  find  from  the  observa- 
tions recorded  on  the  free  balloons,  L'Aerophile 
in  France  and  the  Cirrus  in  Germany,  which  on 
March  2ist,  1892,  and  July  yth,  1894,  reached  the 
same  height  of  10  miles.  In  the  case  of  the 
French  balloon,  the  temperature  descended  to 
104°  F.  below  zero,  and  at  the  same  rate  the  cold 
of  space — viz.,  minus  461°,  would  be  reached  at  a 
height  of  about  30  miles. 

The  ordinary  rate  of  decrease  is  in  general 
about  i°  in  320  feet  after  we  rise  above  the  first 
100  feet. 


106  THE   STORY   OF   THE   EARTH'S  ATMOSPHERE. 

From  what  has  been  said  about  the  slower 
rate  at  which  air  saturated  with  vapour  cools  by 
ascent  when  the  temperature  is  high  near  the  sur- 
face than  when  it  is  low,  we  can  readily  under- 
stand why  cumulus  clouds  and  rain  showers  occur 
in  the  daytime  and  in  warm  latitudes  more  readi- 
ly than  at  night  and  near  the  poles.  In  fact, 
since  at  freezing-point  and  at  sea-level  even  sat- 
urated air  would  cool  as  rapidly  as  i°  F.  in  every 
277  feet;  if  it  were  not  for  imported  convection 
systems  and  clouds  there  would  be  very  little  as- 
cent and  precipitation  of  condensed  vapour  at  all 
in  the  arctic  zones. 


CHAPTER   VII. 

THE  DEW,  FOG,  AND  CLOUDS  OF  THE  ATMOSPHERE. 

WHEN  we  gaze  skywards  and  see  the  filmy 
wisps  of  high  cirrus  cloud,  touching  as  it  were 
the  very  vault  of  heaven,  or  when  we  notice  the 
ragged  scud  of  the  approaching  storm,  half  cov- 
ering the  low  hills,  we  are  witnessing  one  of  the 
first  stages  by  which  the  water  of  our  atmosphere 
becomes  visibly  separated  from  its  gaseous  com- 
panions. 

Another  stage  is  manifested  when  the  pearly 
drops  of  dew  gather  on  the  blushing  petals  of  our 
roses,  or  the  rain  drops  from  the  frowning  storm 
clouds.  Still  another  transformation  scene,  and 
the  beautiful  six-rayed  flakes  of  snow  fall  like 
flowers,  scattered  by  an  angel  hand,  and  cover  up 
the  gloomy  earth  with  a  mantle  of  dazzling  white. 

Yet  one  more  strange   scene,   and   from   the 


DEW,  FOG,  AND  CLOUDS  OF  THE  ATMOSPHERE.  107 

fiery  thunder-cloud  white  balls  of  ice  rattle  down 
as  though  from  some  aerial  glacier. 

The  chameleon  character  of  this  same  water 
element  is  indeed  a  most  fortunate  circumstance. 
Imagine  what  a  dull  world  it  would  be  without 
our  gaudy  sunset  cloud  tints.  What  a  desert  if 
it  never  rained. 

•^"It  happens,  however,  that,  unlike  the  other 
gases,  water-vapour  can  undergo  all  its  changes 
within  the  gamut  of  the  temperatures  we  experi- 
ence on  this  planet. 

Solid  at  32°  F.,  liquid  thence  to  212°  F.,  after 
which  it  becomes  gas. 

Moreover,  the  air  is  thirsty  as  it  were,  and  so, 
from  the  liquid  water,  at  all  temperatures,  and 
even  the  solid  ice,  vapour  is  ever  ascending  by 
evaporation  and  rendered  invisible  as  it  passes 
through  the  other  gases. 

There  is  a  limit,  however,  to  the  capacity  of 
air  for  such  a  temperance  beverage,  which,  like 
the  thirst  of  men,  depends  on  the  temperature. 
Thus,  while  a  cubic  foot  of  air  at  zero  F.  can  hold 
but  •£  a  grain  of  vapour,  at  60°  F.  it  can  soak  up 
5 1  grains.  At  80°  F.  as  much  as  n  grains  can 
remain  invisible  in  the  same  space. 

To  give  a  larger  example.  Suppose  a  room, 
20  feet  square  by  10  feet  high  at  60°  F.,  to  be 
supplied  with  vapour  until  it  could  hold  no  more, 
then  the  air  in  such  a  room  would  weigh  304 
Ibs.,  while  the  vapour,  if  it  were  condensed  to 
water,  would  weigh  but  3  Ibs.,  and  fill  three  pint 
measures. 

When  air  can  hold  no  more  vapour  it  is  said  to 
be  saturated,  and  since,  when  it  is  cooled,  it  is  able 
to  hold  less  and  less  water,  it  can,  even  when  un- 
saturated,  be  made  saturated  by  being  cooled 


108  THE  STORY  OF  THE   EARTH'S  ATMOSPHERE. 

down  to  a  point  of  temperature  some  few  degrees 
below.  This  point  is  called  its  dew  point,  and 
depends  partly  on  how  damp,  partly  on  how 
warm,  it  was  at  first. 

When  very  warm  and  moist,  a  very  slight 
lowering  of  temperature  produces  condensation 
into  cloud  and  finally  rain.  Hence  clouds  and 
rain  will  form  easier  in  warm  countries,  though 
other  conditions  may  make  them  more  constant 
in  cold  countries. 

What  we  ordinarily  term  the  dampness  of  the 
air,  is  not  simply  a  question  of  how  much  vapour 
is  present,  since  warm  air  may  hold  more  than 
cold  air  and  yet  feel  drier. 

It  is  determined  by  the  nearness  of  the  dew- 
point  to  the  existing  temperature,  and  this  de- 
pends on  both  the  amount  of  vapour  present  and 
the  temperature  of  the  air  in  which  it  is  dis- 
solved. 

Ordinarily  the  dampness  in  England  is  about 
60  per  cent,  of  what  could  be,  but  in  very  wet 
weather  it  rises  to  90  per  cent.  ' 

Over  the  ocean  it  is  generally  high  both  in 
warm  and  cold  latitudes,  while  in  the  interior  of 
continents  and  deserts  it  is  occasionally  as  low  as 
15  per  cent.  As  we  rise  above  the  earth  towards 
the  level  of  the  lower  clouds  the  dampness  in- 
creases, until,  at  the  cloud  level,  we  reach  dew-  or 
cloud-point,  where  the  air  is  saturated. 

Dew  itself  is  the  moisture  deposited  on  the 
surface  of  bodies  near  the  earth's  surface,  which 
have  cooled  down  by  radiation  below  the  dew- 
point  of  the  surrounding  air. 

Dr.  Wells,  in  1783,  was  the  first  to  offer  this 
explanation,  and  thought  the  moisture  came  en- 
tirely from  the  air  around.  Of  late,  however,  Mr. 


DEW,  FOG,  AND  CLOUDS  OF  THE  ATMOSPHERE.  109 

John  Aitken,  of  Edinburgh,  and  others,  have 
shown  that  a  large  part  of  the  moisture  comes 
from  the  ground  and  the  plants  on  which  it  is 
deposited.  They  are,  in  fact,  constantly  perspir- 
ing like  human  beings.  In  the  day-time  this 
perspiration  is  evaporated  by  the  warmth  and 
carried  off  by  the  winds.  Only  in  the  cool  and 
calm  of  the  night  and  early  morning  does  it  be- 
come deposited  in  drops  of  water. 

Hoar  Frost  is  simply  dew  formed  when  the 
dew-point  happens  to  be  below  the  freezing  point 
of  water. 

Fog,  or  mist,  may  be  termed  a  cloud  of  vapour, 
formed  near  the  ground  or  water. 

Sometimes,  like  dew,  it  is  occasioned  by  the 
earth  parting  with  its  heat  so  rapidly  as  to  cool 
down  a  stratum  of  air,  just  above  it,  below  its 
dew-point,  as  occurs  in  the  quiet,  anti-cyclonic 
weather,  as  it  is  termed,  occasionally  experienced 
in  these  latitudes  in  winter.  Such  fogs  are  usu- 
ally fairly  dry.  Sometimes  it  occurs  with  a  wind, 
when  it  is  wetter  and  warmer.  In  such  cases  it  usu- 
ally comes  off  the  sea,  and  is  due  to  a  warm  moist 
air  from  the  sea  passing  over  a  cool  ground  surface. 

Locally,  mists  usually  form  in  low  river  val- 
leys, where  the  air  is  nearly  saturated  with 
vapour,  rising  from  the  water  or  moist  land. 

It  seems  strange,  but  it  is  nevertheless  true, 
that  the  pretty  white  valley  mist  of  the  country 
is  a  near  relative  of  the  ugly  nauseous  fogs  of  our 
large  cities.  London  fog  is  simply  Thames  river 
valley  mist,  mixed  with  the  smoke  poured  forth 
by  innumerable  chimneys,  which  is  unable  to  be 
carried  off,  owing  to  an  absence  of  wind  above. 
When  we  can  consume  our  own  smoke,  London 
fog,  as  we  know  it,  will  disappear. 


110  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

Fogs  at  sea  occur  most  frequently  in  summer, 
and  especially  near  cold  currents,  as  off  New- 
foundland, where  the  warm  moist  air  off  the  Gulf 
Stream  passes  over  the  cold  Labrador  current  ; 
in  the  Behring  Sea,  where  the  Japan  current  meets 
the  Arctic  ice;  off  Cape  Horn,  &c. 

At  San  Francisco,  the  Pacific  Ocean  breeds  a 
chilly  fog,  which  rolls  up  the  streets,  and  obliter- 
ates the  sun,  even  in  May,  but  it  is  fortunately 
white. 

The  clouds  of  heaven  have  ever  been  an  ob- 
ject of  wonder  and  admiration  to  the  sons  of 
men.  Long  before  Aristophanes  wrote  his  im- 
mortal comedy,  "The  Clouds,"  we  have  numerous 
references  to  the  clouds  in  the  ancient  Scriptures. 
Job  says:  "  He  bindeth  up  the  waters  in  his  thick 
clouds."  "For  he  maketh  small  the  drops  of 
water  ;  they  pour  down  rain  according  to  the 
vapour  thereof."  "Also  can  any  understand  the 
spreadings  of  the  clouds";  while  in  Ecclesiastes 
we  have  a  wonderful  insight  into  the  whole 
scheme  of  water  circulation  in  the  verse  which 
says,  "  All  the  rivers  run  into  the  sea,  yet  the 
sea  is  not  full.  Unto  the  place  from  whence  the 
rivers  come  thither  they  return  again." 
f"*  The  first  person  who  seriously  observed  and 
/  described  the  clouds  was  Luke  Howard,  the 
Quaker,  who  was  first  attracted  to  the  subject  in 


^ 
t? 


.  _ 

Howard  roughly  divided  clouds  into  three  j 
primary  forms  —  stratus,  cumulus,  and  cirrus  —  1 
and  the  same  division  also  roughly  applies  to  I 
their  height  —  low,  intermediate,  high.  —  J 

Of  late  years,  as  our  knowledge  of  their  vari- 
ous forms  and  mode  of  origin  has  increased,  this 
simple  division  has  been  found  inadequate.  The 


DEW,  FOG,  AND  CLOUDS  OF  THE  ATMOSPHERE.  1 1 1 


late  Rev.  Clement  Ley,  Mr.  Ralph  Abercromby, 
Dr.  Hildebrandsson,  Dr.  Vettin,  and  Messrs. 
Ekholm  and  Hagstrom  have  observed  their 
forms,  behaviour,  and  heights,  and  developed 
quite  a  science  of  clouds. 

The  outcome  of  their  researches  is  briefly  sum- 
marised in  the  International  Cloud  Atlas,  which 
has  just  been  published,  as  a  result  of  the  Interna- 
tional Committee  held  at  Upsala,  Sweden,  in  1894. 

Without  going  into  details,  the  following  gives 
a  general  idea  of  the  varieties  and  corresponding 
heights,  beginning  with  those  near  the  surface; 
the  character  of  the  cloud  being  indicated  as  wet 
or  dry  according  to  the  weather  by  which  it  is 
usually  accompanied : — 

VARIETIES  OF  CLOUDS. 


Height  in  feet. 

Name. 

Description. 

Char- 
acter. 

;    I.  Sea-level 

Stratus. 

Elevated  fog,  so-called. 

Dry  and 

up  to  3000. 

wet. 

2.  4500    to") 
6000.       1 
3.  4500    to  f 

Cumulus.     1 
Cumulo-      y 
nimbus. 

Rounded  heap. 
Tower-like  clouds  with 
round    tops   and   flat 

Dry. 
Wet. 

24,000.  J 

J 

bases. 

4.  6400. 

Strato- 

Rolls  of  dark  cloud. 

Dry. 

cumulus. 

3  5-  6400. 

Nimbus. 

Masses  of  dark  formless 

Wet 

cloud. 

6.  10,000    to 

Cirro- 

Fleecy  cloud,  mackerel 

Dry. 

21,000. 

cumulus. 

sky. 

Average  height. 

1   7.   27,000. 

Cirro-stratus. 

Fine  whitish  veil  giving 
halos  round  sun  and 

Wet. 

moon. 

'•     8.  27,000. 

Cirrus. 

Isolated  feathery  white 

Dry. 

clouds. 

112  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

I  have  left  out  one  or  two  purposely  for  sim- 
plicity's sake. 

Pictures  of  i,  2,  4,  6,  8  are  given  in  figs.  (23) ; 
frontispiece,  (i)  and  (2),  (3)  and  (25),  respect- 
ively. ^. 

It  used  to  be  thought  that  clouds  were  simply 
produced  by  the  mixture  of  cold  and  warm  air. 
In  1788  Dr.  Hutton  of  Edinburgh  propounded 
his  celebrated  law  of  mixtures,  which  briefly 
asserted  that,  when  two  masses  of  air  at  different 
temperatures  mingled,  the  colder  air  caused  the 
vapour  in  the  warmer  air  to  condense,  owing  to 
the  lower  temperature  of  the  mixture  not  being 
able  to  support  the  same  amount  of  vapour  in 
solution.  ^__ 

Dr.  Von  Bezold  has  recently  (1890)  investi- 
gated the  question,  and  has  shewn  that  when 
saturated  warm  air  mixes  with  unsaturated  _cold 
air,  more  cloud  will  be  found  than  when  >atu- 
rated  cold  air  penetrates  unsaturated  warm  air. 
These  two  cases  are  successively  illustrated  by 
opening  the  door  of  a  laundry  on  a  cold  day  and 
the  door  to  an  ice-house  on  a  hot  day.  In  the 
former  case  fog  is  at  once  formed,  but  not  in  the  ] 
latter.  s 

On  the  whole,  however,  he  finds  the  effect  of 
mixture  to  be  very  small  and  ineffective.  In  the 
case  of  Nature  it  usually  occurs  when  a  layer  of 
warm  air  overlies  a  layer  of  colder  air.  ^ 

The  shallow  stratus,  cirro-cumulus,  and  cirro-  A 
stratus  clouds  are  partly  due  to  this  action. 

Where  one  current  crosses  another  at  a  differ-     / 
ent  speed  it  raises  waves  or  billows  in  it,  just  as 
when  wind  passes  over  the  sea.     In  such  waves    I 
there  will  be  cloud  at  the  crests  and  clear  spaces 
in  the  troughs.  , — - 


DEW,  FOG,  AND  CLOUDS  OF  THE  ATMOSPHERE.  113 

Mackerel  sky,  or  cirro-cumulus,  and  the  long 
rolls  of  strato  cumulus  which  follow  one  another 
at  the  rear  of  a  storm,  with  showers  and  clear  in- 
tervals of  several  hundred  feet,  are  examples  on 
a  small  and  large  scale  of  such  aerial  billows. 


FIG.  25. — Cirrus  Cloud  (var  Tracto  Cirrus,  1889). 
P.  Gamier,  Boulogne  Observatory. 

\  The  most  frequent  cause  of  clouds,  however, 
/  is  the  cooling,  due  to  expansion  of  air,  which 
'  ascends  either  freely,  or  by  being  forced  up  a 
— j»ountain-slope,  or  drawn  up  an  aerial  eddy. 

When  the  existing  rate  of  decrease  of  tempera- 

I  ture  with  the  height  is  greater  than   i-|  degrees 

per  300  feet,  air  locally  warmed  will  ascend,  and 
/   cool  at  this  rate  until  the  dew-point  is  reached, 

when  vapour  will  be  condensed  and  cloud  formed. 
l  After  this  the  air,  since  it  is  now  saturated,  will 
I  cool  at  a  much  slower  rate — viz.,  about  £°  F.  in 

300  feet. 


114  THE   STORY   OF  THE   EARTH'S  ATMOSPHERE. 

Ascent  after  cloud  level  is  reached  is  easy, 
therefore,  so  long  as  sufficient  moist  air  is  sup- 
plied. 

;• —    Clouds  are  often   found  hugging   mountains 
/  when  the  surrounding  plains  are  clear.  

J-—      In  popular  language  mountains  are  said  toj 
"  attract  clouds."     This,  of  course,  is  literally  in-  / 
correct.     The  clouds  are  due  in  this  case  chiefly/ 
to  the  lower  air  being  forced  up  the  mountain' 
side,  and  cooled  by  expansion  down  to  dew-point. 
r~~      Clouds  also  occur  in  connection  with  cyclones 

I   or  large  storms,  both  in  the  tropics  and  high  lati- 
tudes. 

This  is  because  the  air  in  the  centres  of  such 
movements  is  damp  to  start  with,  and  is  continu- 
ally rising  and  flowing  out  over  the  surrounding 
drier  air.  — 

The  approach  of  such  a  storm  when  miles 
away  is  frequently  heralded  by  the  appearance 
of  tangled  masses  of  the  lofty  cirrus  cloud,  which 
appear  to  converge  to  a  point  below  the  horizon,, 
as  in  fig.  26,  and  which  represent  the  overflow  of 
the  ascending  damp  air  from  the  centre  of  the 
storm.  These  clouds  are  called  the  "  warning 
cirrus,"  because  their  appearance  and  motion 
(from  the  storm-centre,  unlike  the  lower  clouds 
which  move  towards  it)  indicate  the  position  and 
character  of  the  approaching  disturbance.  Soon 
after  this  storm  signal  has  been  hung  out  by  the 
Celestial  weather-bureau,  sheets  of  cirro-stratus 
appear  below  these  cirrus  wisps,  p.  in,  No.  7, 
which  hide  the  sun  and  often  cause  a  large  halo, 
by  refracting  its  rays  through  the  prisms  of  ice 
of  which  they  are  composed.  The  final  act  in 
the  aerial  drama  is  ushered  in  by  the  appearance 
of  high  stratus  and  nimbus,  No.  5,  with  ragged 


DEW,  FOG,  AND  CLOUDS  OF  THE  ATMOSPHERE.  115 

scud  of  stratus,  No.  i,  quite  low  down.  These 
lower  clouds  move  round  and  in  towards  the 
storm-centre,  and  thus  make  a  considerable  angle 
with  that  of  the  cirrus  flowing  out  from  it. 


FIG.  26. 

If  a  storm-centre,  for  example,  bears  S.  W.,  the 
cirrus  will  also  bear  S.  W,  the  cirro-stratus  S.,  the 
low  clouds  S.  E.,  and  the  surface  wind  E.  S.  E. 

The  storm  culminates  with  the  arrival  of  the 
nimbus,  from  which  rain  or  snow  falls,  after 
which  the  clouds  disappear  in  the  reverse  order 
of  their  arrival,  except  that,  owing  to  the  more 
rapid  motion  above,  in  the  direction  in  which  the 
storm  is  moving,  the  upper  clouds  are  blown 
towards  the  front,  and  are  less  prevalent  in  the 
clearing  up  showery  weather  in  the  rear.  Fig. 
(23)  represents  the  upper  sky  as  seen  after  a 
storm,  from  a  height  of  7000  feet,  in  the  Alps.  A 
sheet  of  lower  cloud  or  fog  is  below  the  spectator. 


Il6  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

Fig.  (27)  shows  the  position  of  the  clouds 
round  a  European  cyclone.  The  continuous 
lines  are  the  isobars,  and  the  dotted  lines  iso- 
therms. 

Almost  the  entire  cloud  mass,  it  will  be  no- 
ticed, lies  to  the  front  of  the  central  ring  of  low 
pressure. 

Amongst  special  forms  of  clouds  may  be  men- 
tioned the  table-cloth  of  Table  Mountain  at  the 


FIG.  27. 

Cape,  and  similar  table-cloths  on  Mount  Pilatus, 
the  Rock  of  Gibraltar,  Atlas,  &c.  These  are  all 
formed  by  the  passage  of  a  warm  moist  current 
over  a  cold  mountain  which  condenses  its  mois- 
ture while  it  is  moving  across  the  summit.  When, 


DEW,  FOG,  AND  CLOUDS  OF  THE  ATMOSPHERE.  117 

however,  it  has  passed  beyond  the  mountain,  the 
vapour  cloud  mixes  with  the  warmer  air  around, 
and,  the  vapour  becoming  reabsorbed,  the  cloud 
gradually  tails  off  into  invisibility. 

In  New  Zealand,  when  a  wet  north-wester  is 
blowing  against  the  Southern  Alps,  and  heavy 
rain  is  falling  on  their  western  sides,  a  long  bar 
of  cloud,  due  to  similar  causes,  may  be  observed 
stretching  along  their  summit  ridge,  extending 
some  little  distance  from  it  above  the  eastern 
plains. 

The  eastern  edge  of  this  cloud  roll  remains 
fixed,  though  the  wind  may  be  blowing  through 
it,  as  it  often  is  in  these  cases,  with  hurricane 
violence. 

Cumulus  clouds  indicate  upward  convection 
movements,  and  are  more  frequent  in  summer  and 
warm  countries. 

They  are  frequently  as  tall  and  thick  as  they 
are  broad,  and  often  pierce  up  right  into  the 
cirrus. 

Clouds  of  the  stratus  form,  on  the  other  hand, 
are  usually  very  shallow  compared  with  their 
area,  which  may  extend  for  hundreds  of  miles. 
They  indicate  horizontal  motions,  and  prevail 
mostly  in  winter  and  cold  latitudes. 

Clouds  generally  increase  during  the  day-time, 
and  reach  their  greatest  height  in  the  afternoon 
and  evening. 

The  average  rate  at  which  clouds  move  in- 
creases  with  their  altitude.  This  accords  with  the 
theory  of  the  general  circulation  in  Chapter  V. 

Cirrus  clouds  have  occasionally  been  observed 
to  move  as  fast  as  250  miles  per  hour  ;  but  their 
average  pace  is  more  nearly  80  miles  in  America 
and  40  in  Europe. 


Il8  THE   STORY  OF   THE   EARTH'S   ATMOSPHERE. 

The  average  velocities  for  the  different  cloud 
levels  observed  at  Blue  Hill,  near  Boston,  have 
already  been  given  on  p.  92. 

The  average  heights  of  the  clouds  are  greatest 
in  summer  and  least  in  winter.  Their  average 
speed  is  least  in  the  former  and  greatest  in  the 
latter  season. 

In  fig.  (25)  a  cirrus  cloud  is  shewn,  in  which 
the  delicate  fibrous  nature  of  this  cloud  is  plain- 
ly seen.  The  contrast  between  this  ice  crystal 
cloud,  like  a  puff  of  white  tobacco  smoke,  and 
the  cumulus  in  the  frontispiece,  composed  of 
heavy  water  drops,  is  very  striking. 

The  Festooned  cumulus  in  fig.  (28)  is  a  special 
form  of  cloud  associated  with  thunder  and  hail- 


FIG.  28. — Festooned  Cumulus. 
Sydney,  N.  S.  Wales.    Jan.  18,  1893. 

storms.     It    is  a    kind    of   inverted    cumulus,    in 
which  a  cold,  very  moist  air  is  moving  over  a  hot 


RAIN,  SNOW,  AND  HAIL  OF  THE  ATMOSPHERE.  119 

and  very  dry  air  such  as  frequently  occurs  in  Col- 
orado, Australia,  and  India.  Under  these  circum- 
stances all  the  condensation  occurs  in  the  cold 
layer,  and  none  in  the  lower,  hot  one.  Portions 
of  the  upper  layer  drop  down  in  large  bulbous 
masses  like  water  balloons,  which  burst  like  soap 
bubbles  and  drop  their  moisture  like  water  run- 
ning out  of  a  cask.  Sometimes  this  water  never 
reaches  the  ground,  being  re-evaporated  while 
passing  through  the  hot  air. 


CHAPTER  VIII. 

THE  RAIN,  SNOW,  AND  HAIL  OF  THE 
ATMOSPHERE. 

RAIN  is  the  final  stage  of  condensation  of  va- 
pour back  into  water,  of  which  cloud  is  a  half- 
way stage.  The  mist  which  composes  a  cloud  is 
formed  of  tiny  drops  of  water  about  ^-g^-inch  in 
diameter.  It  used  to  be  a  puzzle  to  explain  how 
these  water  particles  were  sustained,  and  it  was 
at  one  time  supposed  that  cloud  particles  were 
hollow.  We  know  now  that  this  is  neither  neces- 
sary nor  true,  since  very  small  particles  even  of 
gold  will  remain  suspended  for  a  long  time  in  air; 
the  finer  the  particles  the  longer  they  take  to  fall. 
A  slight  upward  motion  of  the  air  is  therefore 
enough  to  keep  them  balanced.  As  condensation 
proceeds  these  particles  grow  larger  by  fresh  coat- 
ings of  water,  and  the  larger  ones  fall  down 
against  the  smaller  and  mingle  with  them  until 
large  drops  from  -fa  to  -^  inch  thick  form,  which 
are  no  longer  capable  of  being  suspended  and  fall 


120  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

to  the  earth.  Snow  forms  when  the  temperature 
at  which  this  further  stage  of  condensation  oc- 
curs is  below  freezing  point.  Every  snow  crys- 
tal is  a  variety  of  a  six-rayed  cluster,  and  is  sim- 
ilar to  the  crystals  of  salts  which  are  precipitated 
from  a  chemical  solution.  No  one  has  watched 
the  formation  of  snow,  but  it  must  be  very  simi- 
lar to  that  of  crystallisation  out  of  a  solution 
which  is  saturated  with  a  chemical  salt. 

Hail,  unlike  the  delicate  snow  crystals,  is 
frozen  water-drops.  Its  frequent  association  with 
thunder-storms  led  to  the  belief  that  it  was 
caused  in  some  way  by  electricity.  This  is,  how- 
ever, found  to  be  untenable  in  the  searchlight  of 
modern  science,  which  shews  that  electricity  is 
mostly  an  effect,  not  a  cause  of  such  mechanical 
disturbances.  It  is  believed,  that  in  such  storms 
the  rain-drops  formed  in  one  part  of  a  storm  are 
carried  upwards  by  powerful  ascending  currents 
(twenty-five  miles  an  hour  is  enough  to  sustain 
large  drops)  into  higher  regions  of  the  atmosphere 
where  they  are  solidified  by  the  excessive  cold, 
and  being  carried  over  with  the  overflow  which 
takes  place  near  the  top,  fall  down  until  they  are 
redrawn  into  the  interior  of  the  storm  and  again 
whirled  up  aloft.  Receiving  alternate  meltings 
and  freezings,  and  growing  larger  with  each  cir- 
cuit they  make  in  the  atmospheric  churn,  they  are 
finally  thrown  out  on  either  side  of  the  storm  cen- 
tre. This  explains  the  fact  that  in  a  travelling 
hailstorm  there  are  two  bands  where  hail  falls  on 
either  side,  while,  under  the  centre,  it  is  often 
found  that  only  rain  has  fallen. 

Hailstones  have  often  fallen  of  enormous  sizes. 
In  1697,  Robert  Taylor  found  hailstones  in  Hert- 
fordshire 14  inches  in  circumference. 


RAIN,  SNOW,  AND  HAIL  OF  THE  ATMOSPHERE.  121 


In  India,  the  writer  remembers  a  hailstorm  on 
the  great  Brahmaputra  river  when  the  hailstones 
cut  holes  through  the  tarpaulin  cover  of  the 
steamer,  which  were  so  large  that  each  one  had 


IAPHICAI,  MAP 


FIG.  29. 

to  be  mended  with  a  separate  patch.  Hailstorms 
are  intimately  connected  with  tornadoes,  and,  like 
these  phenomena,  are  more  frequent  over  flat 
plains  and  in  very  hot  and  moist  summers  and 
countries. 

The    destruction    dealt   by    hail    on    standing 
crops,  vineyards,  and  orchards  has  led  to  means 


122  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

being  proposed  for  its  prevention.  Under  the  old 
idea  of  its  connection  with  electricity,  lightning 
rods  were  erected,  but  without  avail.  In  France 
and  other  countries  shooting  with  cannon  into  the 
forming  clouds  has  been  tried,  but  with  little  suc- 
cess. 

Planting  of  trees  would  be  m:re  effective, 
since  this  would  tend  to  check  the  rapid  heating 
up  of  the  lowest  stratum  of  air,  which  is  one  of  the 
chief  causes  of  tornado  and  hailstorm  action. 

The  general  distribution  of  rain  in  belts  over 
different  areas  of  the  earth's  surface  has  already 
been  alluded  to.  Rainfall,  like  clouds,  is  more 
prevalent  in  mountainous  than  over  flat  countries, 
and  for  similar  reasons,  especially  cooling  by 
forced  ascent  of  air. 

In  the  accompanying  mean  annual  rainfall 
maps  of  England  and  India,  this  will  be  readily 
seen.  In  England  the  heaviest  falls  will  be  ob- 
served to  occur  in  the  mountains  of  Cumberland 
and  Wales,  and  generally  along  the  hilly  country 
of  the  West  and  North.  In  Scotland  and  Ireland 
it  is  the  same.  The  lowest  rainfalls  under  20 
inches  all  occur  on  the  eastern  sides  of  the  coun- 
try. This  difference  is  partly  due  to  the  fact  that 
the  prevailing  and  most  rainy  winds  are  south- 
west and  drop  a  good  deal  of  their  moisture  be- 
fore reaching  the  eastern  parts,  but  even  were 
these  barriers  absent,  the  rainfall  over  the  flatter 
country  on  the  eastern  sides  would  not  be  very 
much  increased.  In  India,  in  like  manner  the 
dark  shading  along  the  Western  Ghats  down  the 
Bombay  coast  and  along  the  Himalaya  shews  the 
influence  of  the  mountains,  the  heaviest  fall  oc- 
curring near  the  north-east  corner  of  the  Bay  of 
Bengal  in  the  Khasia  hills,  which  offer  an  abrupt 


RAIN,  SNOW,  AND  HAIL  OF  THE  ATMOSPHERE.  123 

wall  4000  feet  high  up  which  the  southerly  mon- 
soon winds,  see  fig  (20),  are  forced. 

Chirapunji,  at  the  edge  of  these  hills,  has  the 
largest  rainfall  in  the  world  (about  500  inches), 


••  FIG.  30. 

half  of  which  falls  in  June  and  July.  On  the 
western  side  of  the  Ghats  rain  falls  heavily  up 
to  250  inches  at  Mahalleshwar  on  their  summits, 
while  the  tableland  of  the  Deccan  on  their  east- 
ern lee  side  has  a  scanty  supply  and  is  one  of  the 
areas  liable  to  drought. 

The   greatest   amount   of   rain   in  a  vertical 
direction  occurs  at   altitudes  where   the   lowest 


124  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

cloud  is  thickest,  that  is,  at  about  3000  feet  in 
Europe  and  4000  feet  in  India  above  sea-level. 
Jn  the  interior  of  large  continents  where  moun- 
tain ranges  are  absent,  especially  when  they  lie 


FIG.  31. 

like  Australia  in  the  zone  of  perpetual  high 
pressure  dividing  the  tropical  from  the  polar 
wind  systems,  the  rainfall  tails  off  to  a  very  few 
inches  as  we  go  inland. 

The  accompanying  rain  map  of  Australia 
shews  this  very  plainly. 

Over  the  whole  of  the  lightly  shaded  area  of 
central  and  west  Australia  there  are  less  than 
10  inches  per  annum. 

This  district  can  never  support  a  large  popu- 
lation. 


THE  CYCLONES  OF  THE  ATMOSPHERE.       125 

CHAPTER  IX. 

THE    CYCLONES    OF    THE    ATMOSPHERE. 

EVERY  large  storm  of  the  atmosphere  is  now 
called  a  cyclone,  because  it  is  found  that  the  air 
moves  round  and  in  towards  a  central  area. 

Formerly  the  word  cyclone  was  only  applied 
to  the  rare  but  violent  storms  of  the  tropics, 
while  the  words  hurricane  and  tornado  are  still 
popularly  used  to  signify  small  and  large  storms, 
indifferently. 

The  term  tornado  is  now  applied  by  meteor- 
ologists entirely  to  certain  storms  of  quite  a 
special  class,  differing  from  cyclones  both  in  size, 
mode  of  origin,  and  effects,  and  it  is  to  be  hoped 
that  the  newspapers  will  learn  eventually  to  give 
up  a  habit  which  only  leads  to  confusion. 

A  cyclone  is  a  large  disc  of  nearly  horizontally 
moving  air  circulating  spirally  round  a  central 
area  over  which  the  barometric  pressure  varies 
from  one-fifth  to  as  much  as  three  inches  below 
that  at  its  border. 

The  direction  in  which  the  wind  circulates  is 
the  same  as  that  in  which  the  earth's  surface 
would  appear  to  rotate  in  each  hemisphere,  if  we 
stood  several  miles  directly  above  the  pole  and 
looked  downwards. 

Cyclonic  storms  range  in  diameter  from  20  to 
as  much  as  3000  miles. 

A  tornado,  on  the  other  hand,  consists  of  a 
narrow  column  of  air  varying  in  width  from  20 
feet  to  1400  feet  which  is  rotating  with  immense 
velocity  (up  to  500  miles  an  hour)  round  a 
central  shaft  up  which  it  is  also  ascending  with 


126  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

a  speed  in  some  cases  amounting  to  100  miles 
an  hour. 

A  cyclone  is  an  elephant,  while  a  tornado  is 
a  mouse,  and  they  differ  just  as  much  in  other 
respects  as  these  two  animals. 

Tornadoes  will  therefore  be  specially  referred 
to  in  the  next  chapter,  together  with  whirlwinds, 
waterspouts,  and  thunderstorms,  which  belong  to 
the  same  family. 

Many  poetic  and  graphic  descriptions  of  the 
awful  grandeur  of  a  real  tropical  cyclone  have 
been  given.  All  descriptions,  however,  pale 
before  the  real  thing. 

The  writer  once  experienced  in  Eastern  Bengal 
the  full  violence  of  perhaps  the  most  disastrous 
cyclone  in  regard  to  destruction  of  human  life 
on  record — viz.,  what  is  known  as  the  Backer- 
gunge  cyclone  of  November  ist,  1876.* 

After  several  days  of  unusually  quiet,  muggy 
warmth  and  murky  skies,  lurid  sunsets,  and  a 
general  sense  of  impending  doom,  the  rain  began 
to  fall  in  torrents  and  the  wind  to  rise  as  the 
night  came  on,  until  at  last  I  had  to  pile  up  the 
furniture  against  the  windows  to  prevent  their 
being  burst  inwards.  The  lightning  flashed  un- 
ceasingly, the  thunder  crashed,  the  wind  tore 
past  like  a  raging  fiend.  All  the  elements 
seemed  to  have  broken  loose,  and  one  could 
almost  fancy  that  the  sober  laws  of  physics  were 
having  "a  night  out."  The  very  rarity  of  such 
hurricane  violence  made  it  all  the  more  alarming. 
After  a  night  of  Tartarean  gloom,  mingled  with 


*  June  I2th  of  this  same  year  witnessed  the  heaviest  fall  of 
rain  ever  measured  in  the  world — viz.,  40  inches  in  24  hours, 
at  Chirapunji,  Assam. 


THE  CYCLONES  OF  THE  ATMOSPHERE.       127 

truly  horrible  noise,  morning  broke  sadly  through 
gaps  in  the  rampart  of  furniture,  and  I  awoke  to 
a  knowledge  that  the  plaster  coating  of  my  house 
lay  strewn  all  over  the  compound.  Later  on  I 
learnt  to  be  thankful  nothing  worse  had  occurred, 
when  I  heard  the  awful  news  that  100,000 
natives  in  the  adjoining  province  had  been 
drowned  by  the  storm  wave  which  was  forced 
up  the  Bay  on  to  the  low-lying  islands  of  Dakhin, 
Shabaspore,  and  Noakolly  at  the  mouth  of  the 
giant  Brahmaputra. 

While  cyclones  are  comparatively  rare  in  the 
tropics,  they  are  very  prevalent,  though  fortu- 
nately as  a  rule  in  a  milder  form  in  higher 
latitudes.  North  and  south  of  latitude  35°  con- 
tinual streams  of  small  cyclones  travel  along  the 
borders  of  the  large  permanent  areas  of  low 
pressure  or  polar  cyclones  which  surround  either 
pole. 

Sometimes  one  of  these  streams  passes  over 
us,  in  which  case  we  experience  wet  and  stormy 
weather.  Sometimes  they  take  a  more  northerly 
or  southerly  track. 

Their  place  is  frequently  occupied  by  large 
areas  of  high  pressure  from  which  the  air  flows 
quietly  outwards  to  feed  the  cyclones.  These 
areas  are  termed  anti-cyclones.  Modern  observa- 
tions shew  that  the  air  which  flows  in  towards 
and  up  the  centres  of  the  cyclone  hollows  flows 
out  above  and  pours  down  the  centres  of  these 
anti-cyclone  heaps.  While  the  weather  in  the 
cyclones,  owing  to  the  ascending  damp  air,  is 
cloudy  and  rainy,  the  weather  in  the  anti-cyclones, 
where  it  is  descending,  is  dry  and  clear.  Years 
ago,  until  about  1830,  there  was  little  known 
about  the  course  of  the  winds  in  cyclones, 
9 


128  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

and  ships  which  mostly  experienced  their  full 
fury  were  at  their  mercy  or  the  individual 
caprice  of  their  commanders. 

About  the  beginning  of  the  century,  Capper, 
of  the  East  India  Company's  Service,  announced 
that  the  storms  of  the  Bay  of  Bengal  were  vast 
whirlwinds. 

In  1828  Professor  Dove  of  Berlin,  and  soon 
after  Redfield  of  America,  Reid  of  England,  and 
Piddington  of  Bengal,  developed,  though  with 
much  diversity  of  opinion,  the  memorable  "  Law 
of  Storms." 

The  chief  point  of  this  law  was  the  fact  that 
the  wind  always  circulated  round  the  area  of 
lowest  barometer  in  a  nearly  circular  spiral  (there 
was  much  unnecessary  dispute  on  this  point) 
against  watch  hands  in  the  northern  hemisphere. 

They  also  ascertained  that  tropical  cyclones 
originated  in  a  belt  about  10°  on  either  side  of 
the  equator  and  travelled  thence  polewards  along 
parabolic  paths,  occasionally  crossing  the  tropical 
belts  of  high  pressure,  where  they  were  broken 
on  their  western  sides  and  into  the  north  and 
south  temperate  zones. 

The  accompanying  fig.  (32)  shows  their  gen- 
eral course  under  these  circumstances. 

An  immediate  consequence  of  these  rough 
rules  was  to  enable  the  mariner  to  avoid  what 
was  termed  the  "dangerous  semicircle  "  (/.  e.,  the 
front  half  in  the  direction  of  travel),  and  to  tell 
the  direction  of  the  storm-centre  by  noting  the 
direction  of  the  wind.  A  rule  by  which  this  is 
simply  remembered  was  afterwards  enunciated 
by  Dr.  Buys  Ballot,  the  eminent  Dutch  meteor- 
ologist, thus : 

"  Stand    with   your    hands   stretched   out   on 


THE   CYCLONES  OF  THE  ATMOSPHERE.       129 

either  side  and  your  back  to  the  wind,  then  in 
the  northern  hemisphere  the  centre  of  the  cyclone 
will  be  to  your  left  hand."  In  the  Southern 
hemisphere  substitute  "  right  "  for  "  left." 


FIG.  32. 

Stated  in  this  form  the  incurvature  of  the 
wind  or  its  inclination  to  the  isobars  which  con- 
tour round  the  central  area  is  completely  over- 
looked. 

This  inclination  is  found  to  increase  with  the 
distance  from  the  centre,  and  the  distance  of  the 
storm  from  the  equator,  quite  apart  from  the 
fact  that  on  land  it  is  always  greater  than  at  sea. 


130  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

Thus  in  the  Philippine  Islands  (latitude  14°) 
it  was  found  to  be  62°,  in  the  Bay  of  Bengal  (lati- 
tude 20°)  to  be  57°,  in  the  United  States  40°,  over 
the  Atlantic  30°,  and  in  England  the  late  Rev. 
Clement  Ley  found  it  to  be  about  20°,  while  Cap- 
tain Toynbee  found  it  to  be  as  much  as  30°  for 
the  Atlantic  in  latitude  50°  N. 

Near  the  equator,  therefore,  it  would  be  mani- 
festly unsafe  for  a  mariner  to  trust  to  the  famous 
old  "circular  theory,"  which  made  the  winds 
blow  directly  along  the  isobars,  since  there  the 
centre  of  the  storm,  instead  of  being  directly  to 
his  left  if  he  stood  until  the  wind  blew  directly 
upon  his  back,  would  actually  be  nearly  in  front 
of  him. 

Dr.  Meldrum  of  Mauritius,  who  was  one  of  the 
most  indefatigable  reformers  of  the  old  circular 
law,  mentions  a  case  where  as  recently  as  January 
24,  1883,  the  captain  of  the  ship  Caledonien  delib- 
erately ran  his  ship  straight  into  the  centre  of  a 
storm  by  following  the  old  rule.  The  modern 
rules  now  advise  the  mariner  (i)  to  avoid  running 
before  the  wind,  (2)  to  lie  to  on  the  starboard 
tack  in  the  northern  hemisphere  or  the  port  tack 
in  the  southern.  By  this  means  the  vessel  may 
be  safely  guided  out  of  the  dangerous  vortex. 

Tropical  cyclones  occur  most  frequently  on 
the  western  sides  of  the  N.  Atlantic,  the  N.  and  S. 
Pacific  oceans,  and  the  S.  Indian  Ocean,  also  in  the 
Bay  of  Bengal  and  the  China  Sea,  where  they  are 
termed  Taifuns.  The  months  in  which  they  occur, 
September  and  October  in  the  N.  hemisphere,  and 
February  and  March  in  the  Southern,  are  soon 
after  the  periods  when  the  equatorial  calms  or 
doldrums  which  lag  behind  the  sun  have  reached 
their  extreme  northerly  and  southerly  positions. 


THE  CYCLONES  OF  THE  ATMOSPHERE. 


'3* 


The  air  is  calm  and  full  of  moisture,  and  this, 
combined  with  the  fact  that  they  are  preceded 
and  accompanied  by  torrential  rain,  has  led  to  the 
conclusion  that  they  are  due  to  the  upward  con- 
vection of  damp  air  which  causes  an  indraft 
towards  some  central  area.  The  heavy  clouds 
and  thunder  and  lightning  which  accompany 
them,  fully  bear  out  the  same  view. 

Moreover,  the  energy  supplied  by  the  con- 
densation of  the  vapour  which  allows  the  air  to 
recoup  itself  for  the  loss  due  to  expansion  has 
been  calculated  to  be  sufficient  to  account  for  the 
immense  wind  energies  they  exhibit.  Professor 
Reye  of  America  calculated  that  the  Cuban  cyclone 
of  October  5,  1844,  used  up  in  three  days  473  mil- 
lion horse-power.  Indeed,  when  we  consider  that 
the  air  in  a  cyclone  100  miles  in  diameter  and  a 
mile  high  weighs  as  much  as  half  a  million  ocean 
steamers  of  6000  tons  a-piece,  we  can  hardly 
wonder  at  the  enormous  amount  of  energy  re- 
quired to  kieep  this  in  motion,  at,  say,  40  miles  an 
hour. 

On  reaching  land,  tropical  cyclones  frequently 
break  up.  They  are  nearly  unknown  on  the 
equator  itself. 

Ferrel  .again  proved  to  be  the  Newton,  who 
was  able  to  weave  all  the  disconnected  facts  relat- 
ing to  cyclones  into  a  reasonable  theory  of  cause 
and  effect.  Assuming  an  inflow  towards,  and  an 
upflow  over,  a  given  area,  he  was  able  to  shew 
that  at  some  little  distance  from  the  equator  the 
spiral  rotation  of  the  winds  and  all  the  other 
phenomena  of  a  cyclone  would  follow  from  the 
law  of  inertia  on  a  rotating  sphere  explained  in 
Chap.  V.  In  an  ideal  case,  where  friction  was 
unconsidered,  the  air  would  tend  to  rotate  round 


132  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

.a  central  area,  as  in  fig.  (33).  At  the  centre  the 
pressure  would  be  very  low,  gradually  rising  to  a 
maximum  on  the  line  separating  the  interior  from 
the  exterior  gyrations.  Outside  this  line  the 

gyrations  would 
be  reversed.  The 
interior  region 
would  be  the 
true  cyclone  and 
the  exterior  a 
kind  of  anti-cy- 
clone, usually 
termed  a  peri- 
cyclone.  Where, 
as  in  nature,  the 
air  experiences 
friction,  the 

pressure  near 
the  centre  would 

FIG.  33.  be      moderately 

low,  the  interior 

arrows  would  point  inward  towards  the  centre, 
and  the  exterior  arrows  outwards.  The  vertical 
circulation  of  the  interior  zone  is  simply  shewn  in 

fig.  (34). 

All  the  results  from  theory  agree  with  those 
observed.  For  example,  an  increase  of  the  vio- 
lence of  the  wind  until  it  suddenly  drops  near  the 
centre,  where  in  tropical  storms  the  clouds  also 
disappear  and  the  air  becomes  clear  and  calm  for 
a  space  of  occasionally  20  miles.  This  is  called 
the  eye  of  the  storm,  and  the  course  of  the  air  is 
believed  to  be  that  in  the  accompanying  diagram, 
fig.  (35),  where  the  violent  rotation  produces  such 
a  centrifugal  force  as  to  cause  some  of  the  upper 
air  to  descend  to  fill  the  vacuum.  Though  the 


THE  CYCLONES  OF  THE  ATMOSPHERE. 


133. 


FIG.  34. 


air  is  calm,  the  sea  is  here  of  that  confused  char- 
acter most  apt  to  make  a  vessel  founder. 

The  weather  of  the  tropical  regions  is  con- 
trolled almost  entirely  by  the  regular  daily  and 
seasonal  changes 
produced  by  the 
path  of  the  sun  in 
the  sky  between 
sunrise  and  sun- 
set, together  with 
that  in  its  average 
altitude  between  summer  and  winter,  and  is  scarce- 
ly affected  except  temporarily  by  the  rare  passage 
of  a  cyclone. 

In  the  extra  tropics,  that  is  to  say  from  about 
latitude  35°  to  the  Pole,  the  weather  on  the 
contrary  is  almost  entirely  made  up  of  a  succes- 
sion of  cyclones  and  anti-cyclones.  The  changes- 
introduced  by  these,  completely  dominate  those 
brought  about  by  the  daily  and  seasonal  causes. 

Extra-tropical  cyclones  are  moreover  believed 
to  be  due  to  different  causes  to  those  of  the 


tropics.  Unlike  the  latter,  they  are  most  fre- 
quent in  winter  when  the  lower  air  is  in  a  stable 
condition,  and  they  sometimes  occur  without 
rain.  They  are  supposed  by  modern  specialists 
to  be  for  the  most  part  eddies  in  the  large  upper 
return  currents,  as  they  flow  over  from  the  Equa- 
tor, and  crowd  into  the  narrowing  space  towards 
the  poles.  The  effects  at  the  earth's  surface  are 


134  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

the  same  as  though  the  air  rose  spontaneously, 
since  an  eddy  in  mid-air  causes  the  lower  air  to 
ascend  just  as  a  whirlpool  sucks  down  water  that 
is  drawn  into  its  vortex.  The  air  which  is  thus 
forced  to  rise,  forms  clouds  and  usually  rain,  but 
the  storm  generally  in  such  cases  belongs  more 
to  the  upper  regions  of  the  atmosphere,  and  is 
less  violent  near  the  earth. 

The  movement  of  cyclones  is  quite  different 
from  that  of  the  winds  that  blow  round  their 
centres.  The  latter  may  vary  from  a  gentle 
breeze  to  a  hurricane  of  100  miles,  and  the  centre 
may  remain  stationary,  but  usually  the  cyclone 
itself  moves  over  the  earth  at  a  speed  which 
varies  in  different  localities  and  for  each  disturb- 
ance. 

We  have  already  noticed  the  general  move- 
ment of  cyclones  in  fig.  (32).  In  all  cases  when 
once  they  are  formed  they  appear  to  be  guided 
chiefly  by  the  upper  currents.  In  the  tropics  the 
west  and  poleward  motion  results  from  a  combina- 
tion of  the  lower  westward  moving  trades  and  the 
upper  poleward  moving  return  (or  anti-trade) 
currents  from  the  equator,  but  they  often  move 
here  as  elsewhere  in  very  different  paths,  though 
generally  north  or  south-westward. 

Beyond  the  tropics  they  are  driven  eastward 
by  the  prevalent  west  to  east  winds,  both  above 
and  below,  and  like  eddies  on  a  river  are  carried 
along  by  the  stream. 

They  also  exhibit  a  tendency  to  move  round 
the  anti-cyclone  heaps  which  are  here  chiefly 
forced  down-flows  (just  as  the  cyclones  are  here 
forced  up-flows)  so  as  to  keep  the  anti-cyclones 
on  their  right  in  the  northern  hemisphere.  This 
principle  is  made  use  of  in  forecasting  their 


THE   CYCLONES   OF  THE   ATMOSPHERE.       135 

probable  motions.  There  appears  to  be  little 
known  about  the  movements  of  detached  anti- 
cyclones or  areas  of  fine  weather,  but  they  fre- 
quently shew  a  tendency  to  move  from  E.  to  W. 
as  well  as  from  W.  to  E. 

The  average  speed  of  cyclones  in  different 
parts  of  the  world  has  been  determined  by  the 
late  Professor  Loomis  from  a  very  large  number 
of  cases,  thus  : — 

Average  Speed  of  Cyclones  over  the  Earth. 

United  States 28  miles  per  hour. 

North  Atlantic 18 

Europe 16 

West  Indies 14 

Bay  of  Bengal  and  China  Sea 8 

The  greatest  amount  of  cloud  and  rain  and 
the  highest  temperature  occurs  in  their  front 
halves,  and  their  vitality,  especially  in  the 
tropics,  appears  to  depend  on  the  continuance 
of  condensation  and  rainfall,  which  allows  the 
air  to  flow  readily  up  their  centres. 

The  vitality  and  longevity  of  some  of  these 
storms  is  wonderful.  Thus  a  few  years  ago  Mr. 
H.  Harries,  of  the  English  weather  bureau,  traced 
a  storm  which  started  in  the  Japan  seas  right 
across  the  Pacific,  America,  the  Atlantic,  and 
Europe,  until  it  was  lost  sight  of  in  the  Baltic. 

The  forecasting  of  daily  weather  in  high  lati- 
tudes requires  a  knowledge  of  the  birth  and 
subsequent  movements  of  particular  storms.  In 
the  tropics,  where  the  daily  weather  changes  are 
small,  the  most  important  problem  is  the  pre- 
vision of  average  seasonal  weather.  This  re- 
quires something  more  than  a  mere  knowledge 
of  travelling  cyclones,  and  observes  large  waves 
of  pressure  which  take  months  to  travel  from 


136  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

equator  to  pole,  or  from  west  to  east.  By  this 
means,  seasonal  forecasting  six  months  ahead 
is  carried  on  in  India,  and  a  similar  method 
could  be  applied  to  the  average  weather  of  other 
countries.  The  waves  of  pressure  caused  by  the 
passage  of  cyclonic  storms  compared  with  these 
large  waves  are  like  the  ripples  on  an  ocean 
billow. 

The  passage  of  cyclones  and  anti-cyclones 
introduce  special  winds  which  possess  peculiar 
characteristics,  due  to  origin  and  direction.  The 
sirocco,  of  Italy  and  Greece,  the  leveche  and 
solano  of  Spain,  the  leste  of  Madeira,  the  Kham- 
sin of  Egypt,  the  Kona  of  Hawaii,  and  the  brick- 
fielder  pf  Southern  Australia  are  all  examples  of 
the  wind  in  the  front  half  of  a  cyclone  which, 
coming  from  regions  nearer  the  equator,  is  in- 
variably warm,  and  dry  and  exhilarating,  or  damp 
and  muggy,  according  as  they  have  travelled 
over  sea  or  land. 

The  lassitude  and  irritability  produced  by  the 
solano  has  given  rise  to  the  Spanish  proverb, 
"  Ask  no  favour  during  the  Solano."  The 
Italian  sirocco  induces  similar  weariness. 

At  the  rear  of  the  cyclones  of  the  temperate 
zone  which  travel  from  W.  to  E.  in  both  hemi- 
spheres, the  wind  blows  from  some  polar  direc- 
tion. 

Locally  are  thus  produced  the  cold  "  Nortes  " 
of  Mexico,  the  "blizzard"  of  the  States,  which 
is  accompanied  by  blinding  snow,  the  "Mistral" 
of  the  Rhone  Valley  and  the  Gulf  of  Lyons, 
the  "  pampero  "  of  Mexico,  and  the  "  southerly 
bursters  "  of  Australia. 

The  "  bora  "  of  the  Adriatic  is  of  the  same 
species,  but  possesses  a  peculiar,  penetrating  cold 


THE   CYCLONES   OF   THE  ATMOSPHERE.       137 

by  being  drawn  down  towards  the  cyclonic 
depression  from  a  lofty  plateau  where  it  has 
acquired  great  cold  by  radiation. 

A  peculiar  wind  arises  in  connection  with  the 
motion  of  cyclones  over  mountain  ranges,  called 
in  Switzerland  the  " foehn"  zx\&  in  America  the 
"  chinook."  In  New  Zealand  it  is  locally  known 
as  a  "  hot  north-wester,"  and  it  is  found  to  occur 
everywhere  on  the  lee  side  of  mountain  ranges 
running  athwart  the  paths  in  which  the  cyclones 
travel.  This  wind  is  uncommonly  hot  and  dry, 
and  melts  the  snow  on  the  Alps  in  one  night 
more  than  the  sun  shining  for  several  weeks. 

In  America  the  chinook  blows  on  the  eastern 
side  of  the  Rockies  and  raises  the  temperature 
of  a  long  belt  of  that  part  of  the  country  per- 
manently above  what  it  would  otherwise  experi- 
ence. 

The  way  in  which  this  heat  is  derived  has 
been  revealed  by  a  knowledge  of  atmospheric 
physics,  and  is  really  quite  simple  when,  as  the 
professor  of  legerdemain  is  fond  of  saying,  "you 
know  how  it's  done." 

On  the  windward  side,  the  air  after  it  has- 
reached  cloud  level,  loses  only  about  4/s°  every 
300  feet  it  ascends.  When  it  has  reached  the  top- 
of  the  range  it  has  lost  a  great  deal  of  moisture 
in  the  shape  of  rain,  and  as  it  descends  on  the  lee 
side  as  dry  air,  it  gains  heat  at  the  rate  of  i3/s° 
per  300  feet,  or,  in  other  words,  it  gains  an  extra 
4/5°  for  every  300  feet  it  descends.  Consequently, 
by  the  time  it  reaches  the  lee  valleys  it  appears 
as  a  hot,  dry  wind.  The  higher  the  mountain 
chain  the  hotter  the  wind.  It  used  to  be  thought 
that  the  famous  hot  nor'-wester  of  the  Canterbury 
Plains  in  New  Zealand  derived  its  heat  from. 


138  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

Australia,  but  it  is  found  that  when  it  is  blow- 
ing hot  and  dry  in  Christchurch  it  is  rainy  and 
cool  on  the  western  side  of  the  southern  Alps. 


CHAPTER  X. 

THE    SOUNDS    OF    THE    ATMOSPHERE. 

As  inhabitants  of  this  earth  planet  we  are  far 
more  dependent  for  our  happiness  on  sound,  even 
inharmonious  noises,  than  we  are  inclined  to  ad- 
mit. A  world  without  the  voices  of  men  and 
animals,  without  music  and  song,  wrapped  in 
profound  silence  would  be  insupportable.  And 
of  all  phenomena,  sound  is  one  which  pecul- 
iarly belongs  to  the  atmosphere,  since  while  light 
can  travel  wherever  aether  exists,  even  through 
vacuum,  sound  cannot  exist  apart  from  air.  Every 
sudden  movement  of  the  air  propagates  a  series  of 
waves  from  the  point  of  origin  of  the  motion  sim- 
ilar to  what  occurs  in  water  when  a  boy  throws  a 
stone  into  a  pond. 

A  sudden  meeting  of  two  solid  objects  gives 
a  blow  to  the  adjoining  air  which  suffices  to 
originate  a  series  of  such  air  waves.  These 
waves  differ  from  those  on  the  surface  of  water 
in  one  essential  point — viz.,  that  whereas  in 
water  waves,  the  water  moves  up  and  down, 
while  the  wave  motion  is  propagated  horizon- 
tally ;  in  the  case  of  air  waves,  the  air  moves 
backwards  and  forwards  in  the  same  direction  as 
that  in  which  the  wave  is  transmitted.  The  air 
is  thus  alternately  compressed  and  dilated,  and 
as  such  conditions  travel  forward  in  all  directions 


THE   SOUND   OF  THE  ATMOSPHERE.  139 

from  the  origin  of  the  disturbance,  the  sensation 
of  sound  which  is  produced  when  such  waves 
meet  the  ear  is  propagated  through  considerable 
distances.  When  these  waves  enter  the  human  ear 
they  beat  up  against  a  delicate  plate,  or  tympanum 
as  it  is  termed,  of  hard  skin,  and  cause  it  to  shake 
backwards  and  forwards.  The  movements  of 
the  tympanum  are  passed  on  by  a  series  of  bones 
in  loose  contact,  which  filter  out  irregularities 
and  pass  the  waves  into  a  kind  of  aural  piano 
fitted  with  a  number  of  delicate  filaments  instead 
of  keys,  each  of  which  is  attached  to  a  separate 
nerve.  Upon  this  piano  tunes  are  played,  as 
in  the  case  of  an  ordinary  piano,  while  from  our 
brains  we  experience  the  sensation  of  high  and 
low  notes,  harmonies  and  discords,  just  as  simi- 
lar effects  can  be  produced  on  the  artificial  in- 
strument. We  can  only  distinguish  such  waves 
as  sound,  when  they  follow  one  another  more 
rapidly  than  16  times  per  second,  or  less  rap- 
idly than  38,000.  Waves  exist  beyond  these 
limits,  but  to  us  they  are  inaudible.  A  deep 
bass  voice  causes  about  100,  and  the  highest 
soprano  has  reached  about  2000  waves  per  sec- 
ond. 

All  sounds  travel  at  about  the  same  rate — 
1 1 20  feet  every  second  in  air  of  ordinary  tem- 
perature. Consequently  when  we  hear  thunder 
follow  about  five  seconds  after  a  flash  of  light- 
ning, we  know  it  is  a  mile  distant. 

The  dense  air  near  sea-level  is  a  better  me- 
dium for  transmission  of  sound  than  the  more 
rare  air  at  great  elevations.  Sound  rises  upwards 
easier  than  it  descends,  and  travels  better  through 
damp  than  dry  air.  In  a  balloon  Mr.  Glaisher 
heard  the  noise  of  a  railway  train  at  four  miles 


140  THE   STORY   OF  THE   EARTH'S  ATMOSPHERE. 

high  when  in  the  clouds.  When  clouds  were  far 
below  him  no  sound  was  heard. 

Echoes  or  reflections  of  sound  are  often  a 
very  curious  atmospheric  phenomenon.  To  echo 
the  last  word  spoken  distinctly,  the  reflecting 
surface  must  be  at  least  no  feet  away.  A  full 
sentence  requires  a  much  greater  distance.  A 
dome-shaped  roof  often  produces  a  multiple  echo, 
the  reflected  waves  undergoing  continual  reflec- 
tions between  the  floor  and  the  roof,  until  the 
wave  motion  is  finally  converted  into  heat. 

In  the  Taj  Mahal,  at  Agra,  the  incomparable 
marble  mausoleum,  erected  by  Shah  Jehan  to  the 
memory  of  his  wife,  the  central  dome  gives  a 
beautiful  multiple  echo. 

In  buildings  of  a  paraboloid  form,  such  as  the 
Mormon  tabernacle  at  Salt  Lake  City,  Utah,  the 
slightest  sound,  such  as  a  pin  dropped  at  one  end 
of  the  building,  can  be  heard  near  a  certain  point 
at  the  other  end.  Yet  this  building  can  seat 
11,000  people.  The  Whispering  Gallery  at  St. 
Paul's  is  another  example  of  the  same  kind.  In 
the  first  case  the  sound  waves  are  all  reflected 
toward  the  same  point,  and  therefore  reinforce 
each  other  enough  to  render  their  continued 
effect  audible.  In  the  latter,  their  continued 
reflection  between  the  walls  of  the  gallery 
prevents  the  loss  they  would  usually  experience 
by  spreading  out  in  all  directions. 

Thunder  can  be  heard  at  30  miles,  explosions 
at  100  miles  or  over.  Thus  the  firing  at  Water- 
loo was  heard  at  Dover.  The  sounds  of  volcanic 
eruptions,  however,  have  been  heard  at  immense 
distances.  In  1883  the  eruption  of  Krakatoa,  a 
volcanic  island  in  the  Sunda  Straits,  was  heard 
over  an  area  equal  to  one-thirteenth  of  the  entire 


OPTICAL  PHENOMENA   OF  THE  ATMOSPHERE.  141 

globe.  In  one  direction  the  sounds  were  heard 
at  Rodriguez  in  the  Indian  Ocean,  3000  miles 
away  (they  took  four  hours  to  reach  it),  and  in 
another,  at  Alice  Springs,  in  the  very  centre  of 
Australia.  At  intermediate  points  every  place 
thought  a  vessel  was  firing  distress  guns,  and 
search  was  made  for  the  supposed  vessel  over  an 
area  as  large  as  Europe.  Besides  sounds,  large 
air  waves  were  propagated,  which  expanded  in 
circles  until  they  girdled  the  earth  and  then  con- 
verged upon  the  Antipodes  of  Krakatoa,  whence 
they  were  reflected  back  again  to  Krakatoa,  and 
so  on  no  less  than  seven  times.  Every  recording 
barometer  in  the  world  shews  little  notches  in  its 
record  for  August  27  and  following  days.  Each 
notch  shews  the  passage  of  the  wave  backwards 
and  forwards  from  Krakatoa.  These  waves 
travelled  with  the  same  velocity  as  the  sound 
waves,  and  took  thirty-six  hours  to  perform 
each  circuit  of  the  globe. 


CHAPTER  XI. 

THE  COLOURS  AND  OPTICAL  PHENOMENA  OF 
THE  ATMOSPHERE. 

THE  story  of  our  advance  in  the  knowledge 
of  Light,  like  that  in  most  other  branches  of 
physical  knowledge,  is  one  of  gradual  dispersal 
of  error  and  perplexity,  and  the  dawning  of 
truth  and  harmony. 

Even  the  great  Newton's  emission  theory,  by 
which  light  was  supposed  to  be  due  to  a  kind  of 
bombardment  of  minute  corpuscles,  broke  down 


142  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

when  subjected  to  the  keen  analysis  of  modern 
science,  and  another  generation,  led  by  Huyghens, 
Euler,  Young,  and  Fresnel,  was  required  to  formu- 
late and  develop  the  modern  theory  of  wave 
motion  of  the  invisible  aether  which  surrounds 
and  penetrates  all  matter.  This  theory  of  aether- 
wave  motion  accords  with  all  the  observed  facts, 
and  enables  discovery  to  march  forwards  with 
certainty  and  power. 

Light  and  heat  are  simply  effects  of  the  same 
wave  motion.  When  the  waves  of  aether  are 
between  e^oo-th  and  ^-^nyth  of  an  inch  in  length, 
they  produce  the  effect  of  light  upon  our  eyes, 
and  at  the  same  time  heat  upon  our  faces. 

When  the  rays  of  other  lengths  between  these 
extreme  limits  reach  us,  they  appear  of  certain 
colours  corresponding  to  their  wave-length  or 
position  in  the  so-called  spectrum  which  is  pro- 
duced when  white  light  is  passed  through  a  glass 
prism.  The  longest  waves  produce  red  light,  and 
the  shortest  blue  or  violet,  the  order  of  colours 
corresponding  to  decreasing  wave  length,  and 
therefore  greater  rapidity  of  wave  succession, 
being  red,  orange,  yellow,  green,  blue,  violet. 
White  light  is  made  up  of  all  these  rays  mixed. 
When  these  rays  either  singly  or  mixed,  as  gener- 
ally happens,  come  in  contact  with  air,  or  matter 
floating  in  it,  they  set  up  small  oscillatory  mo- 
tions in  the  tiny  molecules  of  which  it  is  com- 
posed. The  effect  of  these  motions  constitutes  a 
condition  which  we  term  heat  or  light,  according 
as  it  affects  certain  nerves.  A  portion  of  the  ra- 
diant energy  is  used  up  in  this  generous  perform- 
ance, or  in  other  words  is  said  to  be  absorbed. 
Relatively,  more  heat  can  be  derived  from  the 
long  wave  rays  of  red  colour,  and  more  light 


OPTICAL   PHENOMENA   OF  THE   ATMOSPHERE.  143 

from  the  short  wave  rays  of  violet  colour,  but 
both  are  produced  all  through  the  spectrum. 
Heat  and  light,  therefore,  are  simply  effects  of 
the  same  wave  motion  according  as  it  specially 
affects  our  senses  of  sight  or  feeling,  and  they  both 
inseparably  belong  to  the  same  radiant  energy 
of  wave  motion  of  the  aether,  started  by  a  body 
already  in  a  state  of  incandescence  like  our  sun. 
Rays  near  the  violet  end  of  the  spectrum  pro- 
duce the  chemical  action  noticed  in  photography 
besides  being  converted  into  heat  and  light.  Dawn 
and  twilight  have  ever  formed  expansive  themes 
to  the  poet.  "  Rosy-fingered  dawn  "  is  a  familiar 
metaphor  of  the  immortal  Homer.  These  half 
lights  are  the  result  of  reflection  of  the  sun's  rays 
when  below  the  horizon,  chiefly  by  the  small  dust 
and  water  particles  at  great  heights  in  the  atmos- 
phere. The  fingered  appearance  alluded  to  by 
Homer  is  due  to  the  light  passing  between  clouds 
or  mountains  below  the  horizon.  The  reddish 
colours  of  the  clouds  at  both  times  are  chiefly 
due  to  the  selective  scattering  which  is  exerted 
by  the  dust  and  vapour  suspended  in  the  air. 
The  smaller  waves  corresponding  to  the  blue 
rays,  as  we  have  already  remarked  in  Chap.  VI., 
are  more  easily  turned  aside  than  the  larger  ones 
corresponding  to  the  red  rays,  and  this  dispersion 
reaches  its  greatest  effect  when  the  sun  is  shining 
through  a  great  thickness  of  air  at  its  rising  and 
setting.  Consequently  red  rays  predominate  at 
these  times  and  tint  the  clouds  as  they  succes- 
sively receive  its  parting  or  coming  rays.  Occa- 
sionally when  a  sunset  has  disappeared  below  the 
western  horizon  it  is  brilliant  enough  to  cause  a 
second  sunset  on  clouds  near  the  western  horizon 
by  reflection,  just  as  though  it  were  the  sun  itself. 


144  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

When  the  dust  ejected  by  the  volcano  of  Kra- 
katoa  Island  in  1883  had  spread  in  a  layer  above 
50,000  feet  all  over  the  world  we  had  such  bril- 
liant primary  and  reflected  sunsets,  which  often 
lasted  i£  hours  after  the  sun  had  disappeared. 

The  reflection  was  assisted  by  a  peculiar  ac- 
tion called  diffraction,  by  which  white  light  meet- 
ing fine  dust  is  split  up  into  its  coloured  elements, 
just  as  though  it  passed  through  a  glass  prism. 

In  this  way  a  huge  coloured  ring  called  a  corona 
was  produced  round  the  sun.  This  ring  is  blue 
inside,  and  exhibits  thence  all  the  spectral  colours 
in  turn,  ending  with  red  at  its  border. 

Inside  this  ring  a  white  central  glow  was  pro- 
duced even  when  the  sun  was  high  up  in  the  sky. 
When  it  was  setting  this  diffraction  glow  became 
pink  and  finally  red  through  the  extinction  of  all 
other  colours  except  the  reds  owing  to  the  great 
length  of  air  traversed  by  the  rays.  Such  glows 
are  always  present  to  some  extent,  due  to  dif- 
fraction by  suspended  water  particles,  but  when 
the  Krakatoa  dust  was  still  in  the  upper  atmos- 
phere they  were  intensified,  and  the  ordinary  re- 
flections prolonged  far  beyond  their  .usual  limits. 

Similar  small  coronse  are  produced  when  small 
clouds  pass  over  the  sun  and  moon.  On  a  small 
scale  they  can  be  frequently  observed  when  we 
look  through  our  eyelashes  at  the  flame  of  a 
candle  or  gas  lamp. 

The  smaller  the  particles  of  cloud  the  larger 
the  corona.  Hence  the  large  corona  seen  round 
the  sun  after  Krakatoa,  called  Bishop's  ring  from 
its  discoverer  in  Honolulu,  showed  that  the  ma- 
terial was  composed  of  very  small  particles. 

A  halo  is  a  large  ring  seen  when  the  sun  and 
moon  shine  through  a  thin  sheet  of  cirrus  or 


OPTICAL   PHENOMENA  OF   THE   ATMOSPHERE.  145 

cirro-stratus,  and  can  only  be  produced  by  re- 
fraction through  ice-prisms.  Consequently  its 
presence  is  one  indication  of  the  ascent  of  vapour 
into  very  lofty  regions,  such  as  occurs  in  cyclones. 
It  is  thus  a  signal  of  the  approach  of  rainy  and 
stormy  weather. 

A  primary  halo  is  always  the  same  size — 45° 
diameter.  Sometimes,  however,  secondary  halos 
are  formed  by  more  complicated  refractions  and 
reflections  of  light  through  the  ice  prisms. 

For  example,  outside  the  ordinary  halo,  and 
concentric  with  it,  an  extraordinary  halo  is  occa- 
sionally seen  of  90°  diameter.  Intersecting  these 
halos,  a  huge  circle  passing  though  the  sun  and 
parallel  to  the  horizon  makes  its  appearance.  At 
the  points  of  intersection  of  these  halos,  the  light 
is  so  reinforced  that  the  patches  look  like  sepa- 
rate suns,  and  form  what  are  termed  mock-suns  or 
parhelia.  Similar  appearances  round  the  moon  or 
mock-moons  are  termed  paraselenae.  At  the  oppo- 
site points  of  the  sky  similar  mock-suns  are  occa- 
sionally formed.  Some  years  back  the  author 
saw  four  mock-suns  at  the  same  time.  Two  in 
front  where  the  primary  halo  intersected  the  large 
horizontal  halo,  22^°  on  each  side  of  the  sun,  and 
two  behind  him,  making  angles  of  157^°  with  the 
sun  on  each  side. 

The  mirage,  or  serab  (illusion),  as  the  Arabs 
term  it,  is  a  phenomenon  which  has  often  formed 
a  subject  for  the  poet  as  well  as  the  artist. 

The  thirsty  traveller  in  the  dreary  and  parch- 
ing wastes  of  the  Sahara  and  Arabian  deserts 
frequently  sees  looming  up  in  the  distance  a 
beautiful  lake  dotted  over  apparently  with  islands 
and  trees.  This  lake  is  an  illusion  produced  by 
the  bending  or  reflection  of  the  light  that  occurs 


146  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

at  the  boundary  of  two  strata  of  air  of  different 
temperatures.  In  this  case  a  layer  of  cool  air 
overlies  one  of  very  hot  air  just  above  the  heated 
sand.  Any  object,  such  as  a  tree  or  mound  above 
this  layer,  has  its  image  inverted  by  reflection, 
while  the  light  from  the  ground  is  thrown  back 
by  what  is  termed  internal  reflection.  Conse- 
quently the  effect  is  just  the  same  as  though  a 
layer  of  water  were  really  present.  A  special  kind 
of  mirage  is  termed  "  looming."  In  this  case 
objects  which  are  ordinarily  below  the  horizon 
are  seen  raised  above  it,  sometimes  inverted  and 
sometimes  erect.  These  effects  are  due  to  a  great 
increase  in  the  ordinary  refraction  which  takes 
place  near  the  horizon,  due,  probably,  to  a  cold 
and  dense  layer  of  air  over  the  sea,  overlain  by  a 
warmer  layer  derived  from  the  neighbouring 
land.  The  famous  Fata  Morgana  or  castles  of 
the  witch  Morgana  of  Reggio  are  an  instance  of 
this  kind  of  mirage.  During  certain  conditions 
of  the  air  the  inhabitants  of  Reggio  see  castles 
and  men  and  trees,  etc.,  suspended  above  the  sea 
in  the  direction  of  Messina,  whose  reflected  image 
they  really  are.  A  southern  imagination  con- 
verts them  into  enchantments. 

A  curious  effect  of  looming  occurred  once  at 
Malta,  where  the  top  of  Etna  appeared  by  refrac- 
tion like  an  island  in  the  sea.  Several  ships  sailed 
out  to  take  possession  of  this  supposed  new  island, 
but  soon  the  image  vanished  and  the  quest  was 
seen  to  be  vain. 

This  story  was  paralleled  more  recently  when 
the  gorgeous  Krakatoa  sunsets  first  made  their 
appearance  in  America.  A  local  fire  brigade  in 
a  raw  Western  township,  seeing  the  sky  so  red, 
with  more  zeal  than  wisdom  harnessed  up  and 


OPTICAL   PHENOMENA  OF  THE   ATMOSPHERE.  147 

set  forth  with  all  speed  to  put  it  out.  When 
they  ultimately  found  out  their  mistake  they 
were  not  a  little  put  out  themselves. 

In  the  polar  regions,  where  the  sea  is  usually 
colder  than  the  air,  the  images  of  objects  below 
the  horizon  are  frequently  reflected  to  the  ob- 
server from  the  top  warm  layer  and  appear  in- 
verted. If  the  upper  warm  layer  is  of  no  great 
thickness,  there  is  thus  often  both  a  direct  and 
inverted  image.  Scoresby  once  recognised  his 
father's  ship,  the  fame,  by  observing  its  inverted 
image  through  a  telescope.  The  real  ship  was 
afterwards  found  to  have  been  thirty-five  miles 
distant. 

The  rainbow  has  always  been  a  majestic  sym- 
bol of  the  union  between  earth  and  heaven. 

Iris,  the  goddess  of  the  rainbow,  was  one  of 
the  most  graceful  of  the  Grecian  deities.  She 
was  represented  as  the  messenger  between 
Olympus  and  his  earthly  subjects. 

According  to  the  Teutonic  mythology  the 
rainbow  was  the  bridge  over  which  the  heroes 
passed  to  the  festive  abode  of  Walhalla. 

Robbed  of  its  fanciful  mysticism,  the  rainbow 
loses  nothing  of  its  beauty  when  we  know  that  it 
is  the  result  of  the  refraction  of  the  white  light 
from  the  sun  as  it  enters  the  raindrop  subse- 
quently reflected  from  the  back  of  the  drop  to 
our  eyes.  The  whole  operation  is  so  wonderful. 
The  different  coloured  rays  which  make  up  the 
white  ray  when  they  meet  the  new  surface,  part 
company  according  to  their  wave  frequency,  and 
travelling  along  separate  paths  are  reflected  by 
the  mirror  back  as  though  they  were  painted  in 
the  sky.  The  tiny  violet  waves  being  more  bent 
inwards,  appear  inside  the  bow,  while  the  longer 


148  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

red  waves  form  the  external  boundary.  Ordina- 
rily the  earth  cuts  off  the  lower  half  of  the  bow, 
and  when  the  sun  is  more  than  40°  above  the 
horizon,  the  entire  phenomenon  disappears. 

Inside  the  bow  the  violet  is  occasionally  seen 
repeated  in  what  are  termed  supernumerary  bows, 
while  the  external  bow  is  often  visible  in  which 
the  colours  are  reversed.  The  explanation  of 
these  belongs  rather  to  a  book  on  optics.  The 
"  Spectre  of  the  Brocken  "  is  simply  a  shadow  of 
the  spectator  projected  on  to  a  screen  of  vapour 
rising  up  from  the  surrounding  valleys,  and  may 
be  seen  on  any  mountain  where  the  conditions 
are  favourable. 

The  "  ignis  fatuus,"  or  wandering  flame  occa- 
sionally seen  in  marshy  land,  or  over  church- 
yards, where  it  is  called  the  "corpse  candle,"  is 
believed  to  be  merely  a  distillation  from  the  soil 
of  phosphoretted  hydrogen  gas  which  has  the 
property  of  self-ignition  on  emerging  into  the 
atmosphere. 

The  "aurora  polaris "  or  "northern  lights" 
are  a  manifestation  of  quiet  electrical  discharge 
round  either  pole,  attaining  its  greatest  brilliancy 
and  frequency  near  the  magnetic  poles,  which  are 
at  some  distance  from  the  true  geographic  poles. 
In  the  northern  hemisphere  the  belt  of  greatest 
frequency  (80  auroras  per  annum)  occurs  from 
latitude  50°  to  62°  in  America,  and  from  latitude 
66°  to  75°  over  Siberia.  From  thence  they  dimin- 
ish both  north  and  south. 

The  Aurora  exhibits  various  forms.  Stream- 
ers, curtains,  bands,  and  rays,  and  it  frequently 
coruscates,  whence  the  name  "  Merry  Dancers." 
It  is  believed  that  the  Aurora  is  a  sheet  of  rays 
which  converge  downwards  toward  the  magnetic 


WHIRLWINDS,    ETC.,    OF  THE   ATMOSPHERE.    149 

axis  of  the  earth,  a  kind  of  luminous  collar,  the 
top  of  whose  arch  is  as  much  as  130  miles  above 
the  earth,  though  parts  of  it  are  believed  to  be 
quite  near  the  earth.  It  is  therefore  an  electrical 
discharge  taking  place  in  highly  rarefied  air  or 
vacuum.  Lemstrom  of  Finland  recently  suc- 
ceeded in  causing  an  artificial  aurora  by  suitably 
imitating  what  is  believed  to  occur  in  Nature. 
The  Aurora  is  certainly  closely  connected  with 
the  magnetic  condition  of  the  earth  and  also  of 
the  sun.  When  any  great  sun-spot  appears  on 
the  latter  orb,  the  magnetic  balance  of  the  earth 
is  affected,  as  shewn  by  the  irregular  movements 
of  the  magnetic  needles  and  the  simultaneous 
appearance  of  aurorse  at  both  poles. 


CHAPTER   XII. 

WHIRLWINDS,    WATERSPOUTS,    TORNADOES,   AND 
THUNDERSTORMS    OF    THE    ATMOSPHERE. 

BESIDES  the  large  cyclones,  there  is  a  peculiar 
group  of  local  disturbances  or  storms  of  the  at- 
mosphere which,  according  to  their  violence,  occur 
in  one  or  other  of  the  above  forms.  The  harm- 
less dust-whirl  we  see  arise  on  a  still  day  in  early 
summer,  and  sweep  across  the  young  corn,  is  but 
the  embryo  of  the  terrible  tornado  of  the  Middle 
United  States. 

The  dreaded  simoom  of  the  Arabian  Desert  is 
simply  a  larger  whirlwind  laden  with  the  dust  of 
the  desert.  Where  the  whirl  is  broader  and  high- 
er, and  the  air  is  moist,  we  have  the  common 
thunderstorm  of  Europe  with  or  without  hail,  the 


150  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

"nor'-wester"  of  India,  the  "pampero"  of  the 
Argentine,  and  the  so-called  "  arched  squall  "  and 
"bull's  eye  squall"  of  the  tropical  seas. 

When  the  action  is  very  intense  and  concen- 
trated, we  have  the  "  tornado  "  which  is  common 
in  the  Mississippi  Valley.  The  freaks  of  some  of 
these  tornadoes,  while  generally  of  the  tragic  or- 
der, occasionally  border  on  the  ridiculous.  Thus 
— even  in  India  where  they  occasionally  occur  in  a 
mild  form — it  is  stated  that  in  the  district  of  the 
Brahmaputra,  on  March  26,  1875,  after  a  tornado 
had  passed  the  village  of  Uladah  a  dead  cow  was 
found  stuck  in  the  branches  of  a  tree  some  30 
feet  from  the  ground. 

In  America,  in  the  tornado  of  June  4,  1877,  at 
Mount  Carmel,  Illinois,  the  spire,  vane,  and  gilded 
ball  of  the  Methodist  Church  were  carried  fif- 
teen miles  to  the  north-eastward.  In  other  cases 
ploughshares  and  even  houses  (generally  of  wood) 
have  been  carried  up  into  the  air,  and,  so  to  speak, 
transplanted.  In  the  recent  terrible  visitation  at 
St.  Louis,  in  June  1896,  it  was  stated  that  a  car- 
riage was  lifted  from  the  road  up  into  the  air  and 
gently  let  down  again  100  yards  off  without  dam- 
age, while  at  the  end  of  this  remarkable  perform- 
ance the  coachman's  hat  was  declared  to  have  re- 
mained securely  attached  to  his  head.  This  last 
circumstance  sounds  extreme,  but  there  is  no  ob- 
vious exaggeration  in  that  given  by  one  spec- 
tator who  informed  the  writer  that  he  looked  up 
a  street  in  St.  Louis  and  saw  everything — horses, 
carriages,  people,  and  furniture  being  whisked 
along  in  tumultuous  chaos  towards  him  as  the 
centre  of  the  tornado  passed  over  it. 

When  the  centre  of  a  tornado  passes  it  seems 
to  sweep  everything  movable  along  with  it,  often 


WHIRLWINDS,    ETC.,   OF  THE   ATMOSPHERE.    151 

destroys  the  most  substantial  buildings  and  cuts 
a  clear  lane  through  a  forest.  In  all  these  cases, 
the  prime  cause  appears  to  be  a  local  instability 
of  the  air  due  to  an  aggregation  of  heat  near  the 
surface,  combined  with  an  incursion  of  cold  air 
in  the  stratum  above.  These  together  cause  a 
rapid  fall  of  temperature  in  a  vertical  direction. 

In  such  a  case  even  dry  air  may  temporarily 
ascend  in  a  narrow  column  and  burst  through  the 
upper  layers. 

When  once  this  has  taken  place  the  surround- 
ing air  rushes  in  to  supply  its  place,  and  there 
ensues  a  whirling  round  just  as  in  the  case  of 
water  running  down  through  a  sink. 

In  Tornadoes  the  whirling  round  of  the  air  is 
not  due  as  in  that  of  the  large  cyclones  to  the 
deflection  caused  by  the  rotation  of  the  earth,  since 
this  would  be  practically  insensible  for  move- 
ments within  such  limited  areas.  It  is  due  to  the 
rapid  development  of  gyration  as  the  air  is 
forced  inwards  towards  the  centre  when  once 
such  gyration  has  started.  The  slightest  devia- 
tion to  one  side  of  the  direct  path  to  the  centre 
is  enough  to  start  a  gyration,  and  any  slight 
irregularity  in  the  flow  suffices  to  cause  a  devia- 
tion. 

After  the  whirling  has  once  started,  the  gyra- 
tions near  the  centre  become  so  rapid  that  ulti- 
mately a  funnel  shaped  column  of  highly  rarefied 
air  is  produced,  which  is  marked  by  the  appear- 
ance of  a  sheath  of  cloud  or  water  within  which, 
in  extreme  cases,  is  a  nearly  complete  vacuum. 
Round  and  up  the  sides  of  this  the  air  ascends, 
flows  out  above,  and  again  quietly  descends  over 
a  wider  area. 

When  the  air  is  dry,  the  action,  as  we  know, 


152  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

cannot  continue  very  long,  since  the  uprising  air 
soon  reduces  the  vertical  temperature  differences 
below  i°  in  180  feet.  Dust  whirls  and  sand 
storms  are  consequently  short-lived  and  never  of 
destructive  violence. 

When,  however,  the  lower  air  is  very  damp  as 
well  as  hot,  the  action  can  go  on  for  a  much 
longer  time  and  with  far  greater  energy. 

Lieut.  Finley  of  the  U.  S.  Navy,  who  has  made 
a  special  study  of  American  tornadoes,  estimates 
that  the  velocity  of  the  wind  rotating  near  the 
centre  of  a  tornado  may  reach  as  much  as  500 
miles  an  hour,  and  exert  a  pressure  of  250  Ibs.  to 
the  square  foot.  Even  the  upward  velocity  near 
the  vortex  probably  amounts,  in  many  cases,  to 
over  loo  miles  an  hour,  otherwise  it  could  not 
sustain  the  objects  it  visibly  does. 

The  awful  effects  frequently  produced  by  the 
arrival  of  such  a  piece  of  what  may  be  termed 
•meteorological  dynamite  can  therefore  be  understood. 
The  central  column  of  rarefied  air  by  reason  of 
its  expansion  is  cooled  below  dew  point.  Hence, 
whatever  vapour  exists  there,  becomes  condensed 
into  a  visible  sheath.  This  is  the  cause  of  what 
are  termed  waterspouts,  which  are  only  a  mild 
form  of  tornado.  In  the  real  tornadoes,  the  black 
funnel  shaped  cloud,  which  forms  one  of  their 
most  marked  features,  is  due  to  the  same  causes. 
The  popular  notion  of  a  waterspout  accounts  for 
the  water  by  imagining  it  to  be  drawn  up  from 
the  sea.  But  this  is  erroneous.  When  water- 
spouts pass  over  the  sea,  they  cause  a  disturbance 
and  slight  upward  rise  round  their  bases,  but  the 
long  visible  column,  often  half  a  mile  in  length, 
which  dips  down  from  the  clouds,  is  entirely  com- 
posed of  vapour,  condensed  out  of  the  inflowing 


WHIRLWINDS,   ETC.,   OF   THE  ATMOSPHERE.    153 

air.  As  Ferrel  puts  it  "  the  cloud  (or  rather  the 
conditions  which  favour  the  production  of  cloud) 
is  here  drawn  down  towards  the  earth  by  the  re- 
duction of  pressure  produced  by  the  rapid  whirl- 
ing of  the  air." 

At  the  same  time,  the  downward  dip  is  only  an 
apparent  and  not  a  real  descent  of  water.  As 
long  ago  as  1753,  indeed,  the  great  Franklin  cor- 
rectly explained  this  where  he  says — 

"  The  spout  appears  to  drop  or  descend  from 
the  cloud  though  the  materials  of  which  it  is  com- 
posed are  all  the  while  ascending,  for  the  moisture 
is  condensed  faster  in  a  right  line  downwards,  than 
the  vapours  themselves  can  climb  in  a  spiral  line  up- 
wards" 

The  freshness  of  the  water  in  a  marine  spout 
is  clearly  testified  to  in  a  story  quoted  by  Prof. 
Davis  in  his  Meteorology  : 

A  waterspout  had  fallen  upon  a  vessel  and 
poured  its  contents  so  freely  over  the  captain, 
that  he  was  nearly  washed  overboard.  He  was 
asked  afterwards,  rather  jocularly,  if  he  had 
tasted  the  water  ?  "  Taste  it,"  said  he,  "  I  could 
not  help  tasting  it.  It  ran  into  my  mouth,  nose, 
eyes  and  ears."  Was  it  then  salt  or  fresh  asked 
his  querist  ?  "  As  fresh,"  said  the  captain,  "  as 
ever  I  tasted  spring  water  in  my  life." 

Waterspouts  occur  mostly  in  the  tropics,  and 
during  the  day  hours.  They  are  children  of  the 
sunshine. 

The  prevailing  funnel  shape,  tapering  down- 
wards, of  the  waterspout  or  tornado  cloud,  is  a 
consequence  of  the  increased  pressure  of  the  air 
near  the  surface.  Above  the  surface  the  absence 
of  friction  and  the  lower  pressure  allows  the  cen- 
tral area  of  rarefaction,  produced  by  the  rapidly 


154  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

whirling  air,  to  extend  for  some  space  laterally. 
Lower  down  the  centrifugal  tendency  of  the  ro- 
tating air  is  met  by  increased  inward  pressure 
and  is  thus  confined  to  a  narrower  space.  Out- 
side the  central  core  the  air  moves  gently  towards 
the  centre.  When  water  in  a  basin  is  descend- 
ing through  a  hole,  a  similar  gentle  flow  may 
be  observed,  the  rapid  whirling  only  extending 
for  a  short  distance  immediately  around  the  hole. 

Even  in  destructive  tornadoes  the  area  of 
dangerous  damage  and  violent  wind  is  confined 
to  comparatively  narrow  limits.  The  width  of 
the  destructive  path  of  the  tornadoes  in  America 
has  been  found  by  Finley  to  vary  from  20  feet 
to  about  2  miles,  the  average  being  about  1369 
feet. 

The  length  of  their  paths  is  usually  not  more 
than  20  miles,  since  the  forces  which  give  rise  to 
them,  unlike  those  of  cyclones,  depend  entirely 
on  specially  marked  vertical  gradients  of  tem- 
perature which  seldom  prevail  simultaneously  over 
large  areas. 

The  mode  in  which  the  air  travels  up  into  and 
round  these  phenomena,  may  be  gathered  from 
the  adjoining  Fig.  (36).  Instead  of  rising  up 
vertically  it  travels  along  the  lines  which  are 
represented  as  winding  spirally  round  the  funnel 
until  it  becomes  cooled  partly  by  ascent  and 
partly  by  expansion  into  the  tornado-core  and  its 
vapour  becomes  visible  at  a  point  C  considerably 
below  the  ordinary  cloud  level  FH. 

Tornadoes  may  be  regarded  as  a  kind  of  at- 
mospheric eruption  analogous  to  those  by  which 
the  volcanic  energy  of  the  earth's  interior  is  ex- 
pended in  one  spot. 

They  prevail  where  the  local  conditions  favour 


WHIRLWINDS,   ETC.,   OF  THE  ATMOSPHERE.    155 

the  establishment  of  explosive  heat  conditions. 
For  example,  where  the  geographical  conditions 
are  favourable  to  the  facile  movement  of  cold  air 
from  the  north  alongside  or  above  warm  air  from 
the  south. 

Such   an   area   exists  par  excellence  over  the 
flat   river   basins   of   the    Mississippi,    Missouri, 


Fio.  36.— Tornado  funnel  cloud. 

and  Ohio.  The  states  lying  in  these  basins  are 
those  in  which  tornadoes  are  found  to  be  most 
prevalent. 

The  general  north  and  south  trend  of  the 
mountains  and  hills  in  America  favours  the  flow 
of  air  of  such  contrasted  conditions,  while  the 
prevalent  east  and  west  ranges  in  the  old  world 
make  them  act  as  preventive  barriers. 

The  time  of  year  most  favourable  to  the  pro- 
duction of  tornadoes  is  Spring  or  early  Summer, 
when  the  earth  is  heating  up  rapidly,  and  the  air 


156  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

above  it  is  still  cold  from  the  effects  of  the  pre- 
ceding winter.  Lieut.  Finley  found  May  to  be 
the  month  of  greatest  frequency  of  tornadoes, 
while  during  autumn  and  winter  they  are  almost 
absent.  The  time  of  day  at  which  they  mostly 
occur  is  in  the  afternoon  when  the  accumulation 
of  heat  in  the  lower  layers  has  reached  its  great- 
est amount.  When  the  gun  is  loaded  it  only  re- 
quires the  slightest  pull  on  the  trigger  to  release 
an  immense  potential  of  energy.  Half  a  degree 
more  temperature  and  the  tornado  is  born  and 
starts  off  on  its  wayward  journey. 

The  destruction  caused  by  these  tornadoes  in 
America  is  hardly  realised  in  Europe  which  is  so 
happily  exempt  from  them.  At  the  same  time 
the  deaths  from  this  cause  in  the  U.  S.  are  esti- 
mated to  be  less  than  those  caused  by  fire  and 
flood. 

Thunderstorms,  like  tornadoes,  originate  from 
the  uprise  of  a  mass  of  warm  moist  air,  but  the 
width  of  the  column  of  uprising  air  is  much 
greater,  and  the  whole  action  is  much  less  con- 
centrated and  violent. 

The  vertical  anatomy  of  a  thunderstorm  is 
shewn  in  Fig.  (37)  where  the  spectator  is  sup- 
posed to  be  standing  to  the  right  and  viewing  an 
advancing  storm.  First  of  all  he  sees  a  layer  of 
cirro-stratus  cloud  (c)  *  commonly  in  the  western 
sky  in  the  afternoon.  Gradually  this  grows 
thicker,  and  from  its  under  surface  festoons  (/) 
similar  to  those  in  the  "  Festooned  Cumulus," 
Fig.  (28),  appear. 

*  This  layer  should  extend  as  far  again  as  the  width  of  the 
figure  to  the  right.  The  exigencies  of  space  have  necessitated 
its  curtailment  in  the  adjoining  figure. 


WHIRLWINDS,    ETC.,    OF  THE  ATMOSPHERE.    157 

The  cirro-stratus  may  extend  from  10  to  50 
miles  in  advance  of  the  storm.  In  this  way  as 
soon  as  they  are  visible,  thunderstorms  may  read- 
ily be  forecasted  within  a  few  hours  by  experts 
such  as  the  late  Rev.  Clement  Ley.  Then  follow 
the  thunderheads  (/)  of  cumulo-nimbus  (as  in 
frontispiece)  which  represent  the  front  portion  of 
the  uprising  current.  Below  these,  a  low  level 


FIG.  37. — Thunderstorm  in  section. 

base  (b]  of  similar  cloud  is  seen,  underneath  which 
is  a  rain  curtain  (r).  A  ragged  squall  cloud  (s) 
rolls  beneath  the  dark  cloud  mass,  a  little  behind 
its  forward  edge,  and  the  whole  structure  moves 
over  the  land  at  the  rate  of  from  20  to  50  miles' 
an  hour.  As  the  squall  cloud  comes  overhead 
the  wind  changes  suddenly  from  an  in-flow  to  an 
out-flow,  represented  in  the  figure  by  two  arrows 
near  the  surface,  with  heads  to  the  right.  In  con- 
trast with  the  hot  muggy  air  preceding  the  storm, 
this  squall  is  deliciously  cool,  especially  in  a  Ben- 
gal north-wester.  Simultaneously  with  the  arrival 
of  the  squall,  the  barometer  rises  about  ^V-h  °f 
an  inch,  the  rain  or  hail  begins  to  fall,  the  light- 
ning flashes  and  the  thunder  crashes  right  over- 
head, until  the  centre  passes,  and  everything 
gradually  resumes  its  former  aspect,  except  the 
temperature  which  has  been  permanently  low- 


158  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

ered.  The  outflowing  squall  is  believed  to  be 
very  similar  to  the  recoil  of  a  gun  when  it  is  dis- 
charged. The  humid  air  in  the  centre  of  the 
storm  expands  so  suddenly  in  rising,  that  it  actu- 
ally kicks  against  the  surface  air,  and  drives  it 
outwards  in  the  direction  where  the  pressure  is 
least,  that  is  towards  the  front  of  the  storm. 

'  Thunderstorms  travel  along  with  the  move- 
ment of  the  air  near  their  tops,  while  the  preced- 
ing inflow  in  front  occurs  as  in  the  figure  in  the 
contrary  direction.  This  has  given  rise  to  the 
saying  that  they  travel  against  the  wind. 

There  are  at  least  two  kinds  of  thunderstorms. 
One  is  chiefly  confined  to  the  equatorial  regions 
and  the  summer  in  high  latitudes,  and  the  other 
occurs  in  connection  with  cyclones  in  their  south- 
east quadrants. 

They  are  both  due  to  the  convectional  ascent 
of  warm  moist  air,  but  in  the  former  case  it  is 
locally  manufactured  during  the  daytime.  In 
the  latter,  it  is  often  imported  from  a  distance. 
In  the  former  storms,  the  cloud  is  isolated  and 
continuous  often  from  1000  feet  up  to  the  cirrus 
level  at  30,000  feet.  When  it  ceases  to  ascend 
it  spreads  out  in  a  sheet  in  all  directions,  so  that 
a  thunderstorm  cloud  of  this  kind  often  pre- 
sents in  the  distance  the  appearance  of  a  huge 
anvil. 

The  cyclonic  thunderstorms  are  not  so  de- 
pendent on  local  sun  heat,  and  frequently  occur 
at  night,  and  in  the  winter  season  in  Scotland, 
Norway  and  Iceland.  In  this  case  the  cooling  of 
the  upper  air  produces  the  same  effect  as  the 
heating  of  the  lower. 

Like  the  tornadoes  they  travel  mostly  east- 
wards, and  their  occurrence  generally  betokens 


WHIRLWINDS,    ETC.,    OF   THE  ATMOSPHERE.    159 

the  existence  of  a  cyclone  centre  to  the  N.  W.  in 
Europe,  and  to  the  S.  W.  in  Australasia. 

As  long  ago  as  1752,  Franklin  proved  by  his 
memorable  kite  experiment  at  Philadelphia,  the 
identity  of  lightning  with  electricity  artificially 
produced  on  the  earth.  There  is,  however,  still 
very  little  known  as  to  the  exact  cause  of  the 
accumulation  of  electrical  potential  which  finds 
vent  in  the  lightning  discharge. 

The  air  is  ordinarily  found  to  be  charged  with 
a  certain  amount  of  positive  electricity,  while  the 
earth  is  usually  negative.  The  concentration  ob- 
served in  thunderstorms,  is  believed  to  be  due 
to  the  increase  in  electrical  quantity,  and  rapid 
increase  in  electric  potential  (or  power  of  do- 
ing work)  caused  by  the  masses  of  damp  air 
which  rise  up,  form  towering  cumulus  clouds, 
and  discharge  their  vapour  in  drops,  by  conden- 
sation. 

As  the  tiny  droplets  of  vapour  in  the  cloud 
unite  to  form  single  large  water  drops,  the  elec- 
trical charges  which  always  exist  to  some  degree 
on  their  surfaces,  become  added  together.  Not 
so  the  surfaces;  since  the  surface  of  a  single 
globe  is  always  smaller  than  that  of  two  globes 
which  unite  together  to  form  it.  Consequently, 
as  more  and  more  droplets  unite  together,  the 
electricity  has  less  room  over  which  to  spread 
itself.  It  consequently  increases  in  thickness,  or 
in  electrical  language,  density.  It  takes  300 
trillions  of  droplets  to  form  a  single  rain  drop, 
and  it  thereupon  results  that  the  surface  of  the 
rain  drop  is  one  8-millionth  of  the  area  made  up 
of  all  the  surfaces  of  its  component  droplets. 
Therefore  the  density  of  electricity  on  the  re- 
sulting raindrop  is  8  million  times  increased  and 


l6o  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

by  a  simple  electrical  law  its  potential  or  power 
to  discharge,  is  increased  50  billion  times. 

We  can  thus  understand  how  it  is  that  so  long 
as  masses  of  damp  air  are  ascending  in  sufficient 
quantity  to  cause  the  great  condensation  and 
rainfall  which  usually  accompanies  thunderstorms, 
the  tremendous  discharges  of  lightning  may  be 
produced  and  accounted  for  without  recourse  to 
any  special  theory  of  its  origin. 

Lightning  destroys  about  250  persons  per 
annum  in  America  chiefly  between  April  and  Sep- 
tember. 

Lightning  conductors  act  by  equalising  the 
flow  of  electricity  between  the  air  and  earth  and 
preventing  a  disruptive  discharge. 

They  are  now  generally  made  of  iron  and 
must  always  be  in  contact  with  damp  earth  since 
they  act  not  by  drawing  the  atmospheric  elec- 
tricity down,  but  by  allowing  the  earth  electricity 
to  flow  upwards.' 

Even  in  perfectly  clear  weather  there  is  a  con- 
stant difference  of  electrical  condition  between 
the  air  and  earth.  In  flying  kites  at  Blue  Hill 
near  Boston  with  steel  wire,  a  conductor  has  to 
be  attached  to  the  earth,  otherwise  the  observ- 
ers even  on  a  cloudless  day  experience  severe 
shocks. 

Lightning  is  of  various  kinds.  Sometimes  it 
branches  out  in  all  directions  from  cloud  to  cloud 
and  is  too  far  above  the  earth  to  strike  through 
the  intervening  space.  This  frequently  happens 
in  the  tropics  where  the  author  has  often  wit- 
nessed a  beautiful  electrical  storm  right  over- 
head, the  thunder  of  which  was  inaudible.  At 
other  times,  especially  in  cyclonic  thunderstorms, 
it  occurs  in  lower  clouds  and  strikes  down  to 


WHIRLWINDS,   ETC.,   OF  THE  ATMOSPHERE.    161 

earth  in  what  is  termed  forked  lightning  accom- 
panied by  loud  thunder.  Thunder  is  produced 
by  the  rapid  heating  and  expansion  of  air  by  the 
discharge  passing  through  it. 

The  noise  is  occasioned  precisely  in  the  same 
way  as  the  sudden  generation  and  expansion  of 
gas  which  ensues  upon  the  ignition  of  gunpowder 
in  a  confined  space  such  as  a  gun. 

The  destruction  of  a  tree  or  house  is  occa- 
sioned in  like  manner  by  the  expansion  of  air  or 
material  which  is  unable  to  conduct  the  dis- 
charge. Upon  a  human  being  the  effect  is  partly 
caused  by  heat  and  partly  by  shock  to  the  nerv- 
ous system. 

A  peculiar  form  of  lightning  is  occasionally 
witnessed  in  which  it  descends  from  the  clouds  in 
a  globular  form. 

These  isolated  globes  of  electricity  play  pecul- 
iar pranks,  meandering  slowly  along  in  the  most 
wayward  and  capricious  manner,  and  apparently 
doing  little  damage  until  they  burst.  They  are 
believed  to  be  somewhat  of  the  nature  of  Leyden 
jars  in  which  a  layer  of  air  takes  the  place  of  the 
glass. 

St.  Elmo's  fire  is  an  appearance  sometimes 
seen  on  the  masts  of  ships  in  stormy  weather. 
Each  mast  head  is  surrounded  by  a  faint  lumi- 
nous ball  of  electric  light.  It  is  really  a  brush 
discharge  which  takes  place  between  the  top  of 
the  mast  and  the  highly  charged  atmosphere  over- 
head. 

The  most  violent  storms  of  lightning  and 
thunder  in  the  world  are  probably  to  be  found  in 
the  north  westers  of  Bengal  where  the  lightning 
is  continuous  for  more  than  an  hour  at  a  time. 
This  is  due  to  the  enormous  condensation  caused 


162  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

by  the  upward  convection  of  the  very  damp  air 
of  that  region.  The  most  awe-inspiring  electri- 
cal manifestations,  however,  frequently  occur 
when  a  thunderstorm  occurs  in  a  region  like  Col- 
orado where  the  air  is  usually  dry.  The  author 
once  experienced  a  storm  at  the  Colorado  Springs 
railway  station  in  which  every  time  a  flash  of 
lightning  appeared,  a  miniature  flash  and  loud  re- 
port were  simultaneously  observed  in  the  tele- 
graph office.  The  wire  of'the  conductor  outside 
was  fused,  and  upon  one  of  the  party  venturing 
out  with  an  umbrella  up  he  returned  declaring  it 
was  raining  lead. 

At  the  summit  of  Pike's  Peak,  14,000  feet  high 
in  the  same  district,  the  observers  in  the  now  dis- 
continued observatory  used  occasionally  to  expe- 
rience most  disagreeable  shocks  even  in  the  sim- 
ple act  of  shutting  the  door,  while  after  walking 
across  the  room  they  could  light  the  gas  with 
their  fingers.  In  Canada,  in  winter  when  the  air 
is  very  dry  and  frosty,  the  same  phenomena  are 
frequently  observed. 

It  was  formerly  supposed  that  thunder  and 
hail  were  unknown  in  the  Arctic  regions,  but  Mr. 
Harries  of  the  Meteorological  Office  has  recently 
shewn  that  they  both  occur  right  up  to  Spitz- 
bergen  and  are  fairly  frequent  in  the  Barents 
Sea.  It  seems  possible  that  the  warm  ocean  cur- 
rents bring  enough  warmth  and  moisture  to  these 
cold  regions  to  cause  the  vertical  instability  of 
the  atmosphere  which  originates  them. 

The  peculiar  arched  appearance  of  the  clouds 
in  norwesters,  pamperos,  and  the  arched  squalls 
of  tropical  seas  and  higher  latitudes  is  simply  an 
effect  of  perspective  caused  by  a  long  roll  of  cloud 
advancing  athwart  the  spectator. 


SUSPENSION   AND   FLIGHT   IN   ATMOSPHERE.   163 

CHAPTER   XIII. 

SUSPENSION    AND    FLIGHT    IN    THE    ATMOSPHERE. 

THE  conquest  of  the  earth  by  man  may  be 
looked  upon  as  tolerably  complete.  The  con- 
quest of  the  air  has  so  far  eluded  all  his  efforts. 
Only  for  short  periods  and  with  great  trouble  and 
risk  has  he  been  able  to  mount  into  the  air  by  the 
aid  of  balloons. 

The  balloon  itself,  old  though  it  may  appear 
to  most  of  us,  dates  back  only  100  years. 

Lichtenberg  of  Gottingen,  in  1781,  was  among 
the  first  to  experiment,  and  made  a  small  balloon 
of  goat-skin,  which  ascended  in  the  air  when  filled 
with  hydrogen.  Thomas  Cavallo,  an  Italian  ref- 
ugee, about  the  same  time  began  by  blowing  soap 
bubbles  filled  with  hydrogen,  and  watching  them 
mount  as  the  school-boy  does  to-day.  Before  he 
got  much  further,  a  step  in  advance  was  made  in 
France  by  two  brothers,  Montgolfier,  who  curi- 
ously enough  started  by  trying  to  make  a  cloud 
of  steam  ascend  in  a  silk  bag.  On  lighting  a  fire 
to  increase  the  "cloud  "  they  accidentally  struck 
on  the  "  hot-air  balloon,"  which  has  rendered  their 
names  famous. 

The  first  human  being  to  actually  ascend  in  a 
balloon  was  Pilatre  de  Rozier  on  Nov.  21,  1783  ; 
but  in  this  case  ordinary  coal  gas  was  employed, 
and  has  ever  since  been  generally  adopted. 

Soon  after  this,  in  1785,  Blanchard  safely 
crossed  the  English  channel  in  a  balloon,  and 
thenceforward  ballooning  came  into  fashion, 
though  at  first  it  was  frequently  attended  with 
mishaps  and  loss  of  life.  The  parachute,  which 


1 64  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

is  now  so  familiar  to  the  world  through  the  re- 
cent beautiful  descents  effected  by  Baldwin,  was 
first  employed  by  Garnerin  on  Oct.  21,  1797.  He 
then  descended  safely  from  a  balloon,  but  experi- 
enced violent  oscillations.  These  are  now  obvi- 
ated by  means  of  a  central  aperture  through 
which  the  imprisoned  air  flows  quietly  upwards. 
The  history  of  the  balloon  ascents  of  Lunardi, 
Tissandier,  Fonvielle,  Gay  Lussac,  Green,  Nadar, 
Glaisher,  and  Coxwell  is  that  of  continual  im- 
provement, success,  and  safety.  Their  voyages, 
particularly  those  of  the  two  last,  have  added 
considerably  to  our  knowledge  of  the  conditions 
of  the  upper  air.  Within  quite  recent  years  great 
strides  have  been  made  in  the  construction  of 
balloons,  chiefly  in  relation  to  their  use  in  opera- 
tions of  war,  by  the  English  military  balloon 
department  at  Chatham. 

The  material  employed  is  oxgut,  which  is  ca- 
pable of  holding  pure  hydrogen  without  leakage. 
Since  pure  hydrogen  is  nearly  2-£  times  as  light  as 
coal  gas,  balloons  filled  with  it  have  greater  buoy- 
ancy and  are  better  fitted  to  withstand  the  de- 
pressing influence  of  the  wind  when  captive.  A 
balloon  of  this  material,  which  contains  10,000 
cubic  feet  of  gas,  weighs  only  170  Ibs.  The  top 
valve  is  made  of  aluminium,  and  a  telephone  con- 
ductor is  arranged  for  communication  between 
the  occupant  of  the  car  and  those  below.  Men 
can  readily  be  seen  at  a  distance  of  two  miles 
from  the  car,  and  general  military  reconnaissance, 
including  photography  can  be  conducted  with 
considerable  accuracy. 

By  the  aid  of  balloons  man  has  certainly  suc- 
ceeded in  attaining  suspension  in  mid  air.  They 
have  not,  however,  aided  him  in  travelling  through 


bUSPENSION  AND   FLIGHT  IN   ATMOSPHERE.   165 

the  air  towards  some  definite  point.  If  he  com- 
mits himself  to  them  he  must  needs  go  nolens 
volens  whither  the  wind  may  carry  him.  Far  from 
having  conquered  the  air  as  he  has  conquered  the 
earth  and  the  sea,  he  has  hardly  more  power  to 
guide  himself  in  a  balloon  than  a  piece  of  straw 
hurled  along  by  a  whirlwind. 

Some  few  years  back  Messrs.  Krebs  and  Re- 
nard  in  France  were  supposed  to  have  solved  the 
problem  of  the  dirigible  balloon  by  means  of  a 
cigar-shaped  balloon  and  a  motor  which  drove  a 
rotary  fan  screw  at  one  end ;  but  though  in  calm 
weather  progress  at  some  few  miles  an  hour  was 
obtained,  it  was  found  to  be  useless  against  the 
wind  which  ordinarily  prevails  at  any  consider- 
able height  above  the  earth's  surface. 

The  late  Prof.  Helmholtz  dealt  a  death-blow 
to  the  practical  realisation  of  the  dirigible  bal- 
loon by  shewing  on  theoretical  principles  that  a 
balloon  could  not  be  driven  against  the  air  at  a 
rate  of  more  than  twenty  miles  an  hour  without 
destroying  its  framework.  To  accomplish  aerial 
locomotion  therefore,  we  must  look  elsewhere. 

From  the  earliest  times  the  flight  of  birds  has 
attracted  the  admiration  and  envy  of  mankind. 

The  ancient  legend  of  Icarus  who  made  a  pair 
of  wings  and  singed  them  off  by  flying  too  near 
to  the  radiant  Phoebus,  was  evidently  based  on 
the  desire  man  has  always  shewn,  to  be  able  to  fly 
like  a  bird. 

As  long  ago  as  1470,  that  "preternatural  gen- 
ius," Leonardo  da  Vinci,  in  the  intervals  of  paint- 
ing the  holy  family,  etc.,  amused  himself  by  plan- 
ning amongst  other  things  flying  machines.  More- 
over, he  appears  from  his  remarks,  even  then,  to 
have  realised  that  the  main  difficulty  to  be  met 


1 66  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

with  apart  from  elevating  and  motive  power,  was 
the  question  of  balance. 

The  recent  accident  by  which  that  enthusiastic 
soarer  Herr  Lilienthal  of  Steglitz  lost  his  life,  oc- 
curred through  his  inability  to  accommodate  his 
balance  to  a  sudden  gust  of  wind. 

The  early  history  of  the  attempts  of  man  to 
fly  is  not  calculated  to  inspire  the  human  race 
with  a  belief  in  its  intuitive  sagacity.  For  the 
most  part  it  is  a  history  of  miserable  failures  and 
fatuous  inability  to  realise  the  feebleness  of  hu- 
man muscular  power.  The  first  serious  attempt 
to  grapple  scientifically  with  the  problem  was 
inaugurated  by  Wenham  in  1866  in  a  paper  be- 
fore the  Aeronautical  Society,  in  which  the  prin- 
ciple of  suspension  by  soaring  as  well  as  flapping 
was  alluded  to. 

Since  that  time  great  progress  has  been  made 
in  the  development  of  what  are  termed  flying 
machines  by  Prof.  Langley  of  Pittsburgh,  Hiram 
Maxim  of  England,  Octave  Chanute  of  Chicago, 
and  Hargrave  of  Sydney. 

In  these  machines  no  attempt  is  made  to  imi- 
tate the  flapping  by  which  birds  mount  into  the 
air,  but  only  of  those  principles  by  which  many 
of  them  are  enabled  to  soar  or  sail  with  out- 
stretched wings  when  sufficient  speed  has  been 
attained. 

Although  it  is  a  fairly  safe  rule  to  follow 
Nature,  exact  imitation  is  by  no  means  in  every 
case  necessary  or  advisable.  Thus,  just  as  in 
travel  on  the  earth's  surface,  it  has  been  found 
more  convenient  to  employ  the  wheel  than  rap- 
idly moving  artificial  legs,  so  in  the  atmosphere, 
it  is  better  from  an  aerial  engineering  point  of 
view  to  analyse  the  compound  movement  of  a 


SUSPENSION    AND   FLIGHT   IN   ATMOSPHERE.   167 

bird's  wing  into  the  two  distinct  elements,  sup- 
port and  forward  propulsion,  and  deal  with  them 
quite  separately.  In  the  case  of  the  bird,  the 
wing  thrusts  backwards,  and  also  acts  as  an  in- 
clined plane,  which,  when  it  is  forced  horizontally 
through  the  air,  converts  the  pressure  into  sup- 
port. In  the  artificial  flying  machine,  the  back 
thrust  is  given  by  the  fan  screw  or  aerial  wheel 
at  the  rear  of  the  plane,  and  the  plane  itself  re- 
mains fixed  at  a  certain  angle. 

The  principle  of  the  inclined  plane  is  strictly 
analogous  to  that  by  which  a  kite  is  suspended 
when  moored  in  a  breeze.  When  the  breeze  fails, 
the  boy  converts  his  kite  into  a  flying  machine 
by  running  with  it,  and  restoring  support  by  the 
relative  breeze  thus  created.  If  we  cut  the 
string  of  the  kite  and  supply  it  with  a  motor  and 
propelling  fan,  it  will  fly  itself  without  the  boy's 
aid,  and  become  a  veritable  free  flying  machine. 
The  kite,  therefore,  is  the  basis  of  the  flying  ma- 
chine. A  flying  machine  is  a  self-propelled  kite. 

There  are  two  actions  of  the  wind  on  a  kite  or 
inclined  plane.  Partly  it  tends  to  make  it  drift  to 
leeward,  and  partly  to  lift  it  upward.  Certain 
birds,  such  as  the  Kestrel  hawk,  shewn  in  fig. 
(38),  the  eagle,  vulture,  and  albatross,  (especially 
the  two  latter),  possess  the  power  of  obviating 
the  tendency  to  drift,  and  of  keeping  themselves 
poised,  or  of  sailing  for  long  periods  without 
flapping  by  the  action  of  the  wind  on  their  wing 
planes.  The  precise  way  in  which  this  is  accom- 
plished is  not  yet  fully  determined.  Maxim 
regards  it  as  effected  by  an  intuitive  utilisation 
on  the  part  of  the  birds  of  local  upward  cur- 
rents which  exist  naturally,  or  else  artificially  up 
declivities. 


1 68  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

The  albatross  of  the  southern  seas  which  the 
author  has  frequently  watched  for  hours  and 
days  together,  undoubtedly  makes  use  of  the 
wind  blowing  up  a  wave  to  restore  its  lift,  after 
it  has  descended  nearly  to  the  surface  of  the 
water. 

Prof.  Langley,  on  the  other  hand,  attributes 
the  suspension  in  both  hovering  and  sailing,  more 


FIG.  38. — Kestrel  hawk  hovering. 

generally  to  a  like  intuitive  adjustment  on  the 
part  of  the  bird  to  certain  rapid  changes  which 
are  found  to  occur  in  the  speed  of  the  wind. 
When  a  strong  gust  comes,  he  slides  down  a  little 
to  meet  it,  and  overcoming  the  back  drift  en- 
tirely by  his  forward  momentum,  is  able  to  utilise 
it  simply  for  lifting  him  vertically  to  the  same 
height  he  was  at  before.  When  the  lull  occurs, 
by  lying  flatter,  he  is  able  in  this  way  to  derive  a 
larger  proportion  of  lift  from  the  lighter  wind, 


SUSPENSION   AND   FLIGHT   IN   ATMOSPHERE.   169 

and  therefore  maintains  nearly  the  same  eleva- 
tion, and  so  on. 

In  the  circular  sailing  so  commonly  seen  when 
vultures  sight  a  piece  of  carrion,  the  inclination 
of  the  wing  planes  is  similarly  increased  on  the 
windward  half  and  decreased  on  the  leeward  half 
of  the  circle. 

The  soaring  and  sailing  of  birds  is  only  pos- 
sible while  the  air  is  in  motion.  Directly  there  is 
a  calm,  even  the  Albatross  is  obliged  to  flap. 

It  is  therefore  only  when  a  wind  is  blowing, 
that  soaring  can  be  exactly  imitated  by  an  intel- 
ligently controlled  flying-machine.  In  any  other 
case  an  artificial  wind  must  be  created  by  means 
of  the  rotating  fan-screw  in  order  to  ensure  sup- 
port, and  the  plane  must  be  kept  constantly  in- 
clined upwards. 

It  will  be  long  before  man  will  be  able  to  gain 
such  a  sense  of  flight  as  to  be  able  to  dispense 
with  the  motor  of  his  flying  machine  and  sail  like 
the  albatross  without  any  apparent  wing  motion, 
but  such  a  sense  will  doubtless  gradually  be  de- 
veloped as  soon  as  he  is  fairly  launched  into  the 
air,  on  what  is  termed  the  motor  aeroplane,  and 
future  generations  will  witness  the  ascent  of  man. 

The  present  position  of  human  flight  stands 
thus.  Mr.  Maxim  has  built  a  large  machine  on 
the  aeroplane  principle,  which  on  being  propelled 
forward,  has  lifted  itself  and  several  people  a  few 
feet  from  the  ground. 

Professor  Langley  has  made  a  small  model 
machine  actuated  by  a  petroleum  motor  which 
has  flown  for  a  considerable  distance  while  the 
motive  power  held  out. 

Mr.  Hargrave  of  Sydney  is  making  a  machine 
but  no  actual  flight  has  yet  been  announced. 


170  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

The  basis  of  this  machine  is  the  so-called  cel- 
lular or  double  plane  kite  of  which  Mr.  Hargrave 
is  the  inventor,  and  which  has  recently  been 
shown  to  be  the  most  efficient  and  stable  kite  yet 
made. 

Though  a  slavish  imitation  of  bird  architec- 
ture has  never  found  favour  with  flying  machin- 
ists, a  study  of  birds,  especially  the  large  soaring 
and  sailing  birds,  shows,  what  the  Duke  of  Argyll 
in  his  "  Reign  of  Law  "  has  so  lucidly  demonstrated, 
that  birds  fly  "  not  because  they  are  lighter,  but 
because  they  are  immensely  heavier  than  the  air. 
If  they  were  lighter  than  the  air  they  might  float, 
but  they  could  not  fly.  This  is  the  difference  be- 
tween a  bird  and  a  balloon." 

Any  machine  to  travel  through  the  air  can 
only  do  so  in  consequence  of  its  superior  momen- 
tum. Consequently  a  flying  machine  must  be 
heavy  in  proportion  to  the  resistance  it  offers  to 
the  air. 

Another  important  point  is  deduced  from  the 
circumstance  that  a  bird's  wing  presents  a  great 
length  (from  tip  to  tip)  and  narrow  width  to  the 
wind. 

For  example,  the  wings  of  that  king  of  flight 
the  Albatross  (Dwnudea  exulans)  measure  15  feet 
from  tip  to  tip  and  only  8  inches  across. 

There  is  a  reason  for  this.  When  a  plane  sur- 
face is  forced  through  the  air,  the  upward  pres- 
sure of  the  air  is  mostly  concentrated  near  its 
front  edge.  If  the  surface  extended  far  back 
from  the  edge,  its  weight  would  act  at  some  dis- 
tance from  the  front  edge.  Consequently  the 
unbalanced  pressure  of  the  air  would  tend  to 
turn  the  plane  over  backwards.  If,  however,  its 
width  were  small,  the  weight  would  act  so  close 


SUSPENSION  AND  FLIGHT  IN  ATMOSPHERE.  171 

to  where  the  resistance  acts  in  the  opposite  direc- 
tion that  the  forces  would  neutralise  each  other 
and  stability  ensue. 

Mr.  Hargrave  has  adopted  this  principle  in  his 
cellular  or  box  kite  in  fig.  (39),  whose  construe- 


FIG.  39. 


tion  is  sufficiently  obvious  from  the  figure  to  ren- 
der detailed  description  unnecessary. 

The  dimensions  in  the  figure  are  in  inches. 
The  length  of  each  cell  (from  right  to  left  in  fig- 
ure) is  30  inches,  and  the  width  and  height  and 
opening  between  are  about  n  inches;  but  these 
dimensions  may  vary,  so  long  as  the  two  cells  to- 
gether form  a  nearly  square  area.  An  important 
feature  of  this  peculiar  tailless  kite  consists  of  the 


172  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

covered-in  sides.  These  ensure  stability  even  bet- 
ter than  two  planes,  bent  upwards  in  V  shape, 
such  as  the  wings  of  the  kestrel  when  hovering, 
and  they  prevent  the  kite  from  upsetting,  very 
much  as  the  sides  of  a  ship  give  it  stability. 

Mr.  Maxim  once  showed  the  advantage  of  such 
side  planes  by  a  simple  experiment,  in  which  a 
piece  of  paper,  when  held  horizontally  and  let 
fall  to  the  floor,  is  seen  to  execute  a  series  of  zig- 
zags in  the  air,  frequently  ending  in  its  complete 
overthrow;  whereas,  when  the  same  piece  of 
paper  is  folded  up  round  the  edges  like  a  boat,  it 
sails  to  the  floor  quite  evenly,  and  in  a  straight 
line.  The  flying  machine  of  the  future  seems  des- 
tined to  be  built  somewhat  after  this  pattern. 

The  prime  problem  is  to  launch  a  stable  aero- 
plane into  the  air, -provided  with  an  engine  and 
screwfan  powerful  enough  to  drive  it  forward  at 
the  velocity  required.  Mr.  Maxim  places  his 
planes  at  a  slope  of  i  in  13,  and  his  practical  ex- 
periments have  shown  that  the  support  gained  by 
the  pressure  of  the  air  on  such  planes  is  more 
than  twenty  times,  and  the  motive  power  of  the 
fans'crew  thirteen  times  what  had  formerly  been 
supposed.  The  engine  which  drives  the  fan  is  a 
very  light  one,  actuated  by  petroleum.  Hargrave 
estimates  the  entire  weight  of  an  engine  to  gen- 
erate 3^  horse  power  at  30  Ibs.  It  is  placed  in 
the  hollow  between  the  two  cells  in  fig.  (3g). 

Prof.  Langley's  recent  experiment  with  his 
model  over  the  Potomac  showed  that  the  elevat- 
ing power  derived  from  such  an  engine  is  suffi- 
cient. The  main  difficulty  will  be  to  ensure  sta- 
bility under  all  conditions,  and  to  accommodate 
the  apparatus  to  the  varying  currents,  by  the  aid 
of  movable  front  and  side  wings.  To  essay  a 


SUSPENSION   AND   FLIGHT   IN   ATMOSPHERE.    173 

journey  except  in  a  dead  calm,  without  consider- 
able practice,  would  at  first  probably  end  in  mis- 
haps. An  era  of  preliminary  misadventure,  in 
fact,  appears  to  be  almost  a  necessary  corollary 
to  the  establishment  of  every  new  form  of  loco- 
motion. That  success,  however,  will  eventually 
be  achieved  is  now  the  firm  belief  of  all  those  who 
have  studied  the  question. 

The  development  of  the  flying  machine  will 
also  be  much  assisted  by  improvements  in  the 
kite.  The  most  efficient  kite  will  be  the  most 
suitable  aeroplane  basis  for  the  flying  machine. 
The  kite  was  first  invented  by  the  Chinese  gen- 
eral, Han  Sin,  in  206  B.  c.,  for  use  in  war,  and  was 
frequently  employed  after  that  date  in  China,  by 
the  inhabitants  of  a  besieged  town,  to  communi- 
cate with  the  outside  world.  After  this  kites  ap- 
pear to  have  degenerated  into  mere  toys. 

At  the  middle  of  the  present  century,  how- 
ever, Pocock  of  Bristol  employed  them  to  draw 
carriages,  and  is  said  to  have  travelled  from  Bris- 
tol to  London  in  a  carriage  drawn  by  kites. 
They  were  also  occasionally  employed  to  elevate 
thermometers  to  measure  the  temperature  of  the 
upper  air,  by  Admiral  Back  on  the  Terror,  and 
Mr.  Birt  at  Kew  in  1847. 

These  observations  had  been  quite  forgotten 
when  the  author  first  suggested  the  employment 
of  kites  for  systematic  observations  in  1883.  It 
has  since  been  discovered  that  Dr.  Wilson  of 
Glasgow,  as  long  ago  as  1749,  resuscitated  kites 
from  their  long  burial  with  a  similar  idea  of  em- 
ploying them  to  measure  temperature. 

In  the  author's  experiments,  steel  wire  was 
first  employed  to  fly  them  with.  Two  kites  of 
diamond  pattern  made  of  tussore  silk  and  bam- 


174  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

boo  frames  were  flown  tandem,  and  four  self- 
recording  Biram  anemometers  weighing  i£  IDS. 
each  were  attached  at  various  points  up  the  wire. 
Heights  from  200  to  1500  feet  were  reached  by 
the  instruments,  and  the.increase  of  the  average 
motion  of  the  atmosphere  was  measured  on  sev- 
eral occasions  for  three  years.  Kites  were  also 
employed,  first  by  the  author  in  1887,  to  photo- 
graph objects  below  by  means  of  a  camera  at- 
tached to  the  kite  wire,  the  shutter  being  released 
by  explosion.  Since  that  time  kite  photography 
has  leapt  into  popularity,  and  has  been  success- 
fully practised  by  M.  Batut  in  France,  Capt.  Ba- 
den Powell  in  England,  and  Eddy  in  New  Jersey. 

The  figure  following  represents  a  recent  pho- 
tograph of  Middleton  Hall,  Tamworth,  taken  by 
Capt.  Powell  with  a  kite-suspended  camera  at  a 
height  of  about  400  feet  above  the  ground. 

At  the  Blue  Hill  Meteorological  Observatory, 
near  Boston,  Mass.,  which  is  carried  on  by  Mr.  A. 
L.  Rotch,  tandems  of  kites  are  used  to  elevate  a 
box  of  self-recording  instruments,  cameras,  etc. 

The  adjoining  fig.  (41)  shows  the  building, 
which  is  630  feet  above  sea  level,  and  a  tandem 
of  Hargrave  kites  supporting  a  camera  with  the 
adjustment  involving  the  use  of  an  extra  cord  for 
slipping  the  shutter,  devised  by  Mr.  W.  A.  Eddy. 
The  height  of  the  camera  is  determined  by  simul- 
taneous observations  of  theodolites  at  the  end  of 
a  base  line. 

By  attaching  several  kites  to  the  same  main 
wire  great  altitudes  have  been  reached  at  Blue 
Hill,  and  complete  records  of  the  pressure  and 
temperature  recorded  on  a  revolving  drum  of  a 
Richard's  thermograph  and  barograph. 

The  highest  point  attained  so  far  was  9385 


SUSPENSION  AND   FLIGHT  IN  ATMOSPHERE.  175 

feet  above  sea  level,  in  October,  1896.  In  order 
to  accomplish  this,  nine  kites  (of  moderate  size) 
and  three  miles  of  steel  wire  were  required.  At 


FIG.  40. 

the  highest  point  the  temperature  fell  to  20°, 
while  at  the  observatory,  8755  feet  below,  it 
was  46°. 

On  other  occasions  when  the  author  was 
present  heights  of  6079  and  7333  feet  were  at- 
tained. 

For  all  such  purposes,  therefore,  kites  are  able 
to  do  as  much  as  free  balloons  up  to  about  three 
miles.  They  are  also  cheaper  and  more  portable 


176  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

than  captive  balloons,  and  possess  far  greater  ele- 
vating power,  especially  in  windy,  weather,  when 
such  balloons  are  nearly  useless. 


FIG.  4i. 


It  was  further  suggested  by  the  author  in 
1888,*  that  kites  could  be  used  for  various  pur- 
poses in  war  as  well  as  science. 

*  Les  Cerf  Volants  Militaires.  Bibliotheque  des  Con- 
naissances  Militaires.  Paris,  1888. 


SUSPENSION  AND  FLIGHT  IN  ATMOSPHERE.   177 

Since  then  Capt.  Baden  Powell,  in  May,  1895, 
read  a  paper  on  "  Kites,  their  uses  in  War."  In 
both  these  publications  it  was  pointed  out  that 
kites  possessed  several  distinct  advantages  over 
balloons;  next,  that  they  could  be  applied  to  all 
the  purposes  for  which  balloons  could  be  em- 
ployed, such  as  signalling,  photography,  torpedo 
projection,  carrying  despatches  between  vessels, 
and  lastly,  they  could  be  employed  to  raise  a  man 
for  purposes  of  reconnaissance. 

This  question  of  "man  raising"  was  long 
scouted  as  impossible,  but  both  Capt.  Powell  and 
Mr.  Hargrave  have  practically  proved  its  possi- 
bility by  elevating  themselves  by  kites,  the  former 
having  reached  a  height  of  100  feet. 

To  give  an  idea  of  the  size  of  kite  required 
for  such  a  purpose,  Capt.  Powell  was  lifted  by  a 
single  large  kite  spreading  500  square  feet,  weigh- 
ing 60  Ibs.,  and  capable  of  folding  into  a  package  12 
feet  long.  Mr.  Hargrave,  at  Stanwell  Park,  N.  S. 
Wales,  on  Nov.  12,  1894,  was  raised  16  feet  by 
four  kites  flown  tandem  which  spread  together 
an  area  of  232  square  feet,  the  wind  blowing 
about  21  miles  an  hour.  The  total  weight  sup- 
ported was  208  Ibs.  An  ounce  of  fact  is  said  to 
be  worth  a  ton  of  theory.  Here  we  see  that  in 
an  ordinary  20  mile  an  hour  wind  a  kite  area 
amounting  to  250  square  feet  is  ample  to  support 
a  man. 

For  a  speed  of  only  10  miles  an  hour  a  larger 
surface  would  be  required,  but  if  the  system  of 
tandem  kites  recommended  by  Hargrave  is  fol- 
lowed, this  could  be  readily  attained  by  the  ad- 
dition of  more  kites.  Under  these  circumstances, 
by  two  or  more  Hargrave  kites  a  man  could  be 
raised,  as  in  fig.  42,  and  effect  a  reconnaissance 


178  THE  STORY   OF  THE  EARTH'S  ATMOSPHERE. 

of  an  enemy's  fortifications  and  dispositions, 
especially  in  mountainous  country,  with  consider- 
able ease  and  far  greater  immunity  than  in  a  cap- 
tive balloon. 

The  portability  of  such  a  series  of  kites  even 
for  man  lifting  may  be  guessed  from  the  remark 


FIG.  42. 


by  Mr.  Hargrave,  in  a  paper  dated  August  5, 
1896,  that  "a  nineteen  square  feet  kite  has  been 
made,  that  weighs  only  19  ounces,  and  folds 
to  about  the  size  of  an  umbrella.  Ten  of  these 
could  be  tucked  under  one's  arm,  and  with  a  coil 
of  line  and  a  decent  breeze,  an  ascent  could  be 
made  from  the  bridge  of  a  torpedo  boat  or  the 
top  of  an  omnibus." 

The  torpedo  boat  certainly  sounds  more  he- 
roic, and  probably  less  dangerous  than  the  om- 
nibus. 

Numerous  possibilities  have  been  suggested 
by  Capt.  Baden-Powell,  and  there  seems  no  rea- 
son why  kites  should  not  enter  in  as  a  regular 


SUSPENSION   AND   FLIGHT   IN   ATMOSPHERE. 


'79 


part  of  the  paraphernalia  of  naval  and  military 
operations. 

Some  few  years  back,  the  author,  with  a  kite 
of  the  ordinary  diamond  pattern,  18  feet  by  14 
feet,  was  able  to  carry  up  600  feet  of  steel  rope 
cable,  by  which  Col.  Templer  tethered  his  large 
war  balloon  in  Egypt. 

This  weighed  50  Ibs.,  and  as  an  additional  test, 
a  man's  kit  weighing  10  Ibs.  was  suspended  to  its 
tail.  Two  such  kites  could  lift  a  man  and  pack 
away  like  fishing  rods. 

Quite  recently  (July,  1896)  a  brochure  by 
Prof.  Marvin,  dealing  with  the  whole  science  of 
kites,  has  been  published  by  the  U.  S.  Weather 
Bureau.  This  represents  the  most  complete 
discussion  of  kite-flying  up  to  date,  and  one  or 
two  of  the  results  are  worthy  of  special  record. 

The  best  kites  are  double  plane  Hargraves, 
with  certain  improvements  in  details.  Tandems 
of  two  kites  only,  with  9000  feet  of  wire  out, 
have  several  times  reached  over  6000  feet  in 
height. 

Kites  can  be  made  to  fly  at  angles  of  60°  or 
more,  and  utilise  most  of  the  wind  pressure  in 
lifting. 

By  adjusting  the  point  of  suspension  or  alter- 
ing the  kite,  we  can  make  it  fly  in  the  ideal  po- 
sition. This  is  found  to  occur  when  the  direction 
of  string  or  wire  is  inclined  at  an  angle  of  66°  to 
the  horizon,  and  cuts  the  kite  plane  at  right 
angles,  so  that  the  latter  is  inclined  at  24°  to  the 
horizon. 

Also  theory  shews  that,  in  order  to  gain  the 
greatest  effect  when  kites  are  flown  tandem,  the 
largest  kite  or  a  bunch  of  two  ought  to  be  placed 
at  the  top  of  the  main  wire. 


180  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

In  conclusion. — By  balloons  alone,  man  will 
never  be  able  to  complete  the  conquest  of  the 
air.  For  travel  through  the  air,  or  as  Prof. 
Langley  terms  it  "  aero  dromics,"  steam  propelled 
kites  will  be  the  future  vehicle.  For  rest  in  the 
air,  it  is  not  impossible  that  kites  will  again  be  a 
serious  rival  of  balloons.  In  fine,  we  may  look 
upon  kites  as  likely  to  take  a  very  much  more 
important  place  in  the  future  than  in  the  past 
story  of  our  atmosphere. 

Before  closing  this  chapter,  it  is  worthy  of 
notice  that  the  principle  of  the  inclined  plane  is 
made  use  of  in  two  other  important  applications 
of  the  motion  of  the  atmosphere  besides  that  of 
supporting  kites — viz.,  in  the  sails  of  ships,  and 
in  windmills. 

In  the  former,  the  wind  meets  the  sail  at  a 
certain  angle,  and  produces  effects  analogous  to 
those  on  a  kite,  especially  when  the  latter  forges 
overhead,  under  the  influence  of  a  freshening 
breeze. 

The  water  here  acts  like  the  controlling  string, 
except  that  it  allows  the  sail  and  boat  to  move 
through  it,  and,  so  to  speak,  form  fresh  attach- 
ments every  instant.  The  slip  to  leeward  is 
analogous  to  the  lift  in  the  kite,  which  is  checked 
by  the  inextensibility  of  its  string.  The  back 
drift  is  prevented  by  the  pressure  of  the  water, 
and  the  shape  of  the  main-sail,  which  tends  to 
make  its  forward  part,  and  therefore  trn  boat, 
turn  continually  towards  the  wind.  The  shape 
of  the  boat,  the  jib-sail,  and  the  action  of  the 
rudder  convert  this  turning-round  force  into  con- 
tinuous motion  ahead. 

As  in  the  case  of  the  kite,  there  is  one  posi- 
tion (different  for  each  combination  of  sails  and 


SUSPENSION  AND   FLIGHT  IN  ATMOSPHERE.  181 

boat,  and  varying  with  the  force  of  the  wind)  in 
which  the  greatest  advantage  or  speed  is  attained 
for  a  given  direction.  To  find  this  and  maintain 
it  is  the  object  of  the  steersman. 

In   practice  it  appears  to  be  very  similar  to 
the  best  inclination   for  a  kite,  so   that  for  any 


FIG.  43.— Yachting  in  Sydney  Harbour. 

wind  between  head  and  beam,  the  sail  should  not 
be  inclined  more  than  24°  to  the  keel.  In  the 
case  of  a  windmill,  "  the  angle  of  weather,"  as 
it  is  termed,  or  the  angle  which  the  sails  make 
with  the  plane  of  rotation,  answers  to  the  angle 
between  the  keel  and  the  boat-sail,  and  varies, 
according  to  circumstances,  round  an  average 
of  24°. 

Windmills   are   a   means    of    converting   the 


1 82  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

motion  of  the  wind  into  mechanical  energy,  which 
may  be  employed  either  for  pumping  up  water, 
grinding  corn,  or,  as  Lord  Kelvin  suggested  in 
1881,  for  generating  electricity.  Before  the 
present  coal-burning  epoch,  windmills  used  to  be 
extensively  employed  for  corn-grinding.  To-day 
they  are  mostly  employed  in  raising  water  for 
drainage,  storage,  or  irrigation.  Most  railway 
stations,  every  farm-house,  and  almost  every  pri- 
vate country  house  in  the  Middle  United  States 
and  Australia,  have  their  windmill  and  tank. 
Labelled  "  cyclone  "  or  "eclipse,"  according  to 
their  particular  make,  they  form  quite  a  feature 
of  the  landscape,  and  it  is  estimated  that  there 
are  more  than  a  million  such  mills  in  the  United 
States  alone. 

The  "useful  efficiency"  of  windmills,  espe- 
cially in  the  modern  geared  form,  is  comparable 
with  that  of  the  best  simple  steam-engines. 

A  geared  modern  wheel,  20  feet  in  diameter, 
will  develop  5  horse-power  in  an  18  mile  an  hour 
breeze,  and  can  be  applied  to  work  agricultural 
machinery  and  dynamos  for  electric  lighting. 
With  a  single  wheel  of  this  size,  Mr.  M'Questen 
of  Marblehead  Neck,  Mass.,  U.  S.  A.,  works  an 
installation  of  137  electric  lights,  for  which  he  for- 
merly used  a  steam-engine.  As  a  result,  he  finds 
that  he  effects  a  saving  of  more  than  50  per  cent. 

According  to  Lord  Kelvin,  wind  still  supplies 
a  large  part  of  the  energy  used  by  man.  Out  of 
40,000  of  the  British  shipping,  30,000  are  sailing 
ships,  and  as  coal  gets  scarcer,  "wind  will  do 
man's  work  on  land,  at  least  in  proportion  com- 
parable to  its  present  doing  of  work  at  sea,  and 
windmills  or  wind  motors  will  again  be  in  the 
ascendant." 


LIFE   IN   THE   ATMOSPHERE.  183 

CHAPTER   XIV. 

LIFE   IN    THE    ATMOSPHERE. 

THE  limits  of  space  warn  us  abruptly  that  we 
must  bring  our  story  to  a  close.  And  yet,  facing 
us  in  the  book  of  nature,  there  is  a  large  unwrit- 
ten story  of  how  the  atmosphere  affects  the  lives 
of  men  and  plants,  embracing  questions  connected 
with  weather,  climate,  disease,  hygiene,  agricul- 
ture, sanitation. 

The  chief  elements  of  climate  have  already 
been  dwelt  upon  in  the  chaper  on  temperature 
and  rainfall. 

Hygiene  and  sanitation  open  out  points  in 
which  other  factors,  such  as  soil  enter  as  well 
as  air. 

The  relations  of  the  atmosphere  to  agricul- 
ture, though  a  subject  of  immense  interest  to  the 
agriculturist,  is  not  a  fascinating  one  to  the  gen- 
eral public.  Prof.  Hilgard,  of  the  University  of 
California,  has  exhaustively  discussed  this  theme 
in  a  bulletin  published  by  the  U.  S.  Weather  Bu- 
reau, 1892,  and  Sir  J.  B.  Lawes  and  Professor 
Gilbert  have  carried  out  experiments  in  England, 
at  Rothampsted,  all  of  which  show  that  in  order 
to  derive  our  maximum  subsistence  from  the  soil, 
we  must  have  a  thorough  knowledge  of  the  ac- 
tions which  take  place  between  it  and  our  atmos- 
phere. 

The  relation  of  climate  to  life,  health,  and 
disease  is  a  very  wide  one,  and  though  it  has  at- 
tracted man's  attention  for  years,  it  has  only  re- 
cently been  studied  with  anything  like  scientific 
accuracy.  An  excellent  summary  of  the  prin- 


1 84  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

cipal  modern   results  will  be  found  in  Moore's 
Meteorology. 

As  an  example  of  how  disease  is  dependent  on 
season,  the  following  table  will  suffice: — 

Development  measured  by.  Mortality. 
Disease  Maximum.  Minimum. 

Enteric  fever Oct.,  Nov.  May,  June. 

Smallpox Jan.  to  May.         Sept.,  Oct. 

Measles June',  Dec.  Mar.,  Oct. 

Scarlet  fever Oct.,  Nov.  Mar.  to  May. 

The  opposition  between  enteric  and  smallpox, 
in  regard  to  season,  shows  clearly  that  seasonal 
conditions  have  a  great  deal  to  answer  for  in  the 
development  of  disease. 

There  is  little  doubt  that  besides  the  regular 
effects  of  seasonal  changes,  the  quality  of  the 
air  of  a  place  is  a  potent  factor  in  relation  to 
health. 

We  talk  of  going  away  for  a  change  of  air,  and 
we  know  that  beneficial  effects  usually  follow  if 
we  choose  our  fresh  locality  aright. 

The  air  of  cities,  as  we  have  seen,  contains 
vastly  more  dust  particles  than  that  of  the  coun- 
try, and  it  is  full  of  other  impurities,  thrown  off 
by  the  multitudes  of  human  beings  crowded  to- 
gether in  a  small  space. 

The  pallor  of  children  in  cities  compared  to 
the  ruddy  health  of  those  who  dwell  in  the  com- 
paratively unpolluted  country  air  is  well  known. 
Similarly  the  air  on  mountains  and  high  plateaux 
is  less  dusty  and  vastly  purer  than  that  near  sea- 
level. 

In  certain  parts  where  vegetation  decays  in 
presence  of  water,  noxious  exhalations  arise 
called  significantly  mal-aria  (bad  air),  and  cause 
fevers  not  only  in  the  Mangrove  Swamps  of  the 


LIFE   IN   THE  ATMOSPHERE.  185 

tropics,  but  formerly  even  in  the  undrained  fen- 
districts  of  England. 

This  bad  air  usually  remains  quite  close  to  the 
ground,  and  its  effects  can  often  be  obviated  in 
the  tropics  by  sleeping  on  an  upper  floor. 

The  atmosphere  undoubtedly  acts  in  many 
cases  as  a  disease  propagator  by  conveying 
germs  from  one  place  to  another. 

For  example  the  mysterious  influenza,  which 
has  of  late  years  so  afflicted  the  whole  world,  is 
evidently  propagated  through  the  air.  As  a  rule, 
however,  water  is  a  far  more  effective  dissemi- 
nator of  disease  than  air,  and  where  a  good 
water  supply  has  been  established,  in  many 
parts  of  India,  where  formerly  cholera  was  rife, 
it  now  occurs  very  rarely  and  in  a  milder  form. 

In  general,  the  atmosphere  acts  as  a  health 
and  life  giver. 

The  more  fresh  air  we  breathe,  the  more  we 
dilute  the  poisons  which  would  otherwise  harm 
our  systems. 

We  are  no  doubt  temporarily  and  permanently 
affected  by  the  particular  climate  we  live  in,  as 
well  as  by  the  air  we  breathe. 

Climate  is  an  average  of  the  general  weather  con- 
ditions, and  is  chiefly  determined  by  the  tempera- 
ture, rainfall,  humidity,  sunshine,  and  winds  which 
prevail  in  a  district. 

All  the  regular  and  irregular  variations  men- 
tioned in  chapters  (IV.)  and  (VIII.)  are  involved, 
particularly  annual  and  daily  temperature  ranges. 

At  some  seasons  a  change  to  a  drier  and 
warmer  climate  such  as  that  of  Egypt  or  Colo- 
rado is  desirable. 

Sometimes  a  mild  one  like  that  of  Madeira  or 
New  Zealand  is  recommended,  while  a  return  to 


1 86  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

England  or  Europe  is  often  indispensable  to  the 
Anglo-Indian  who  has  endured  years  of  Indian 
heat. 

Permanent  residence  in  different  climates 
tends  to  develop  certain  national  characteristics. 

Thus  the  dry,  rapidly  changeable,  continental 
climate  of  North  America,  accounts  for  the  ac- 
tivity and  impulsive  go-aheadness  by  which  the 
Americans  are  characterised.  At  the  same  time 
it  accounts  for  their  liability  to  neuralgia. 

The  debilitating,  nerveless  lassitude  of  the 
natives  of  tropical  coasts  is  directly  due  to  the 
moisture  and  heat. 

The  dry  heat  of  central  India  and  Arabia  de- 
velopes  the  martial  energy  of  the  Sikh  and  the 
Bedouin,  while  the  mild  but  cool  and  temperate 
climate  of  England  and  Western  Europe  is  dis- 
tinctly accountable  for  the  well-balanced  mental 
and  physical  development  of  the  races  which 
have  hitherto  ruled  the  world. 

Climates  may  be  hot  or  cold,  moderate  or  ex- 
treme (i.  e.,  of  small  or  large  range),  dry  or  damp, 
calm  or  boisterous. 

It  was  formerly  deemed  sufficient  to  pay  at- 
tention to  the  temperature  alone,  but  it  has  now 
been  found  that  the  other  factors  are  equally 
important. 

Even  in  regard  to  temperature,  the  average 
for  the  year  is  no  safe  criterion.  The  average 
is  an  artificial  centre,  round  which  the  values 
oscillate,  and  may  be  very  seldom  experienced. 
The  ranges  are  far  more  important. 

Thus  Calcutta,  in  Bengal,  has  the  same  mean 
temperature  of  77.7°  F.,  as  Agra,  in  the  North- 
West  Provinces,  but  their  climates  are  very  dif- 
ferent when  the  ranges  of  temperature  are  con* 


,  LIFE   IN   THE   ATMOSPHERE.  187 

sidered.  The  difference  of  average  temperature 
between  the  hottest  and  coldest  months  at  Agra 
is  34°,  at  Calcutta  only  20°.  The  average  daily 
range  at  Agra  is  about  30°,  at  Calcutta  only  16°. 

When  we  touch  rainfall  and  humidity  we  find 
Agra  has  only  29  inches  to  Calcutta  65  inches; 
while  if  5  represents  the  humidity  at  Agra,  8  rep- 
resents the  amount  at  Calcutta.  Agra  also  has 
half  the  cloud,  and  therefore  about  double  the 
bright  sunshine  of  Calcutta.  Such  instances  could 
be  multiplied  indefinitely. 

Here,  therefore,  we  have  two  places  situated 
in  the  same  river  valley,  only  4°  of  latitude  apart, 
and  yet  with  totally  different  climates. 

To  attempt  to  group  climates  together  over 
large  areas  is  therefore  impossible,  except  very 
roughly. 

The  old  divisions  of  one  torrid,  two  temperate 
and  two  arctic  zones  served  as  a  rough  outline. 
They  are  totally  inadequate  to  explain  the  varia- 
tions found  at  places  not  far  apart  within  the 
same  zone. 

The  only  way  to  gain  an  idea  of  the  climate 
of  a  place,  apart  from  a  study  of  actual  figures, 
is  to  have  a  clear  idea  of  the  effects  of  all  the 
different  factors,  such  as — 

(1)  Latitude. 

(2)  Hemisphere,  north  or  south. 

This  makes  a  great  difference.  The  tempera- 
ture ranges  are  far  smaller  in  the  Southern 
hemisphere. 

(3)  Situation  with  respect  to  large  continents, 
particularly  east  or  west.     If  on  the  east,  as  the 
U.  S.  or  China,  the  temperature  ranges,  daily  and 
seasonal,  are   much   greater    than  on   the  west 
sides. 


1 88  THE   STORY    OF    THE    EARTHS    ATMOSPHERE. 

(4)  Position,  oceanic,  coastal,  or  continental. 
This  affects  both  temperature  range,  and  humid- 
ity very  largely. 

(5)  Elevation   above    the    sea,   and   whether 
isolated    or  on  a  tableland.     If  the  former,  the 
climate  is  moderate;  if  the  latter,  extreme.     In 
both  cases  the   general   temperature  diminishes 
about  i°  F.  for  every  300  feet  of  elevation  above 
sea-level. 

(6)  Situation   with    respect    to    neighbouring 
mountain    ranges,    especially    leeward    or   wind- 
ward, with  reference  to  prevailing  winds.     If  on 
the  windward  side,  such  as  Mull,  Coimbra  in  Por- 
tugal, Vancouver,  Bombay,  Colombo,  Valdivia  in 
Chili,  Brisbane,   and   Chirrapunji    in  Assam,  the 
rainfall    is    often    over    75    inches,  while   corre- 
spondingly  on    the    lee    sides    of   the   adjacent 
ranges  we   find,  Aberdeen,  Salamanca  (less  than 
ten  inches),  Cariboo  (east  of  the  Coast  range), 
Poona,   Bandarawela,  Bahia   Blanca,  Roma,  and 
Shillong,  with  amounts  varying   from   20  to  30 
inches  only. 

(7)  Situation  with  respect  to  prevalent  winds, 
trades,  anti-trades,  monsoons.     This   determines 
the  season  of  rain,  such  as  the  monsoon  rains  in 
the  Indian  summer,  whereas  the  summer  in  Aus- 
tralia, exposed  to  the  trades,  is  the  dry  season. 
The  temperature  conditions  are  thus  consider- 
ably modified. 

(8)  The  neighbouring  oceanic  currents.     The 
effects  of  these  have  already  been  alluded  to  on 
p.  60. 

(9)  The  nature  and  covering  of  the  adjacent 
land. 

(10)  Situation  with  respect  to  the  tropical  or 
circumpolar  rain  and  wind-belts. 


LIFE   IN   THE  ATMOSPHERE. 


189 


As  types  of  various  general  climates  at  sea- 
level,  the  following  may  serve  as  illustrations. 


Type. 


CLIMATES. 

Examples. 


10  • 


Cumana, 


(2)  Tropical,  lat.  10°  to  23°— 


Description. 

Hot,  moist,  equable,  sa- 
lubrious. 


(a)  Coastal, 

r  ,     .                   (  Similar  to  (i)  but  less 
Htg'K'ong,        j       ^able   aU    salubri- 

Jfhore.                }   Hot,  dry,  and  extreme, 

(J)  Inland,         1   M^ndalay,             j"      p**'  excePl  in  win' 

1  Timbuktu,           J       ten 

f  Riviera,                f  Temperate      and     dry, 
(3)  Sub  -  Trop  -      S.  California,               owing  to  position  be- 
ical,  lat.  23°  to  -i  Cape  Colony,      -^       tween     tropical     and 

lat.  35°,              j  Southern    Aus-          polar   rain-belts,  very 

(_      tralia,                 [      salubrious. 

(4)  Temperate  — 

f  England, 

1  Europe, 

Cool,  moist,  and  equable 

(a)  North,   lat.  I  United  States, 
35    to  lat.  6o,1  Central  Siberi'a> 

>•  near  sea,  dry  and  ex- 
treme  inland. 

L      and  China, 

(6)  South,    lat. 
35°  to  lat.  50°, 

New  Zealand, 
Tasmania, 

Cool,  moist,  and  equa- 
ble, most  salubrious 
in  the  world. 

(5)  Polar,      lat. 
60°  to  poles, 

N.  Siberia,           j  Cold  and  fairly  dry,  ex» 
Greenland,           (      treme  inland. 

Judged  by  averages  alone,  a  climate  with  an 
annual  average  temperature  between 

75°  and  85°  is  hot. 
65°    "    75°  is  warm. 
55°    "    65°  is  mild. 

50°    "    55°  is  temperate. 
40°    "    50°  is  cold. 
Below    40°  is  arctic. 


I  po  THE  STORY  OF  THE  EARTH'S  ATMOSPHERE. 

These  adjectives  are,  however,  only  applica- 
ble when  the  range  is  small  between  summer  and 
winter. 

Man  can  never  hope  to  control  or  sensibly 
alter  the  climate  of  the  countries  in  which  he  is 
placed.  Nature  works  on  too  vast  a  scale.  He 
can,  however,  by  studying  the  different  kinds  of 
climate  and  their  properties,  discover  which  are 
suitable  for  certain  diseases  and  ages,  and  by 
utilising  this  knowledge,  to  some  extent  shelter 
himself  against  influences  which  are  recognised 
to  be  hostile,  and  which  lead  not  merely  to  loss 
of  individual  life  and  health,  but  to  degeneration 
of  the  human  race. 


INDEX. 


A. 

Bora,  136. 

Abbe,  Professor,  105. 
Abercromby,  Ralph,  in. 
Actinometer,  36. 
Albatross,  flight  of,  168,  170. 
Ammonia  in  atmosphere,  18,  21. 

Boyle's  law,  95,  98. 
Brickfielder,  136. 
Brocken,  Spectre  of  the,  148. 
Bull's-eye  squall,  150. 
Buys  Ballot's  law,  128. 

Anticyclones,  29,  127,  132,  134. 

Anti-trades,  84. 

C. 

Arched  squall,  150,  162. 
Argon   in  atmosphere,   18. 

Capper,  128. 
Carbonic  acid,  18,  20. 

Atmosphere,      composition,      17; 
electricity,    159;    height,    9,    14; 
history,    9;    laws,    94;    life    in, 

Cavallo,  Thomas,  163. 
Celsius,  39. 
Chanute,  Octave,  166. 

183;    movements    (winds),    64; 

Charles,  law  of,  96,  98. 

optics  of,    141;   origin,   9;   pres- 
sure, 15,  16,  25;  sounds  of,  141; 
temperature,    31  ;     weight,     15, 

Chinook,  137. 
Cirrus,  high.     See  Clouds. 
Clayton,  92. 

16,  25. 

Climate,  183. 

Aurora  borealis,  148. 

Clouds,     99,     106,     119;     cirro-cu- 

mulus,   in;    cirro-stratus,    in. 

B. 

114,    145,    156;    cirrus,    no,    in, 

114,  117,  118,  144;  cumulus,  100, 

Bacilli,  nitrogen  abstracting,  20. 

no,   in,   117,   118;   cumulo-nim- 

Back, Admiral,  173. 

bus,     in;     formation     of,     22; 

Baden  Powell,  177. 

forms    of,    no:    height   of,    111, 

Baldwin,  164. 

118;    high-cirrus,    106;    nimbus, 

Balloons,  15,  88,  93,  163. 

in,    114;    stratus,    no,    ill,   114, 

Ballot,  Dr.  Buys,  128. 

117;   strato-nimbus,   in;  veloc- 

Barometer, 16,  25. 
Barometric    pressure,    variations 

ity  of  movement,  117. 
Coal,  21. 

in,  27,  58,  68,  81. 
Berson,   Dr.,   balloon   ascension, 

Colour  of  the  sky,  25,  102,  141. 
Colours,  sunset,  103. 

15- 

Convection  currents,   104,   158. 

Birds,  flight  of,  166. 

Conservatism  of  energy,  97. 

Birt,  173. 

Corona,  144. 

Blanchard,  163. 
Blizzards,  136. 

"  Corpse  candle,"  148. 
Courant,  ascendant,  59. 

Blue   Hill   Observatory,   92,   118, 

Coxwell,    balloon    ascension,    15, 

1  60,  174. 

164. 

Blueness  of  sky,  102. 

Cumulus.     See  Clouds. 

13                                                                    IQI 

192 


Cumulus,  festooned,  156. 

Halo,  144. 

Currents,  ocean,  46,  60,  no. 

Hann,  63. 

Cyclones,  29,  59,  79,  84,  99,  125. 

Hargrave,  166,  170,  177. 

Hargrave  kite,  170. 

D. 

Harries,  162. 

Davis,  Professor,  153. 

Haughton,  Dr.,  60. 
Hawk,  flight  of,  167. 

Dawn,  143. 

Heat,  31,  97,  142. 

Dew,  106. 
Diffusion  of  gases,  too. 
Disease,  184. 
Doldrums,  82,  130. 
Dove,  Professor,  128. 
Dust  in  atmosphere,  23,  143,  184. 

Hildebrandsson,  Dr.,  in. 
Hot  north-wester,  137. 
Howard,  Luke,  no. 
Hurricanes,  99,  125. 
Huyghens,  142. 
Hypsometry,  30. 

E. 

I. 

Ekholm,  92,  in. 
Electricity    of    the    atmosphere, 

Ignis  fatuus,  148. 
Inclined  plane,  167. 
Inertia,  curve  of,  67. 

Emission  theory  of  light,  141. 
Energy,  conservation  of,  97. 

isobars,  30,  71. 
Isothermals,  44. 

Euler,  142. 

Explosions,  140. 

J. 

F. 

Fahrenheit,  39. 

Japan  current,  60,   no. 
Joule,  98. 
Jupiter,  atmosphere  of,  10. 

Fata  Morgana,  146. 

Ferrell,  48,  60,  63,  66,  74,  131. 

K. 

Finley,   Lieutenant,   152. 

Flying  machines,  88,  93,  163,  165. 

Kelvin,  Lord,  182. 

Foehn,  137. 

Khamsin,  136. 

Fog,  109. 

Kinetic  theory  of  gases,  90. 

Fonvielle,  164. 
Forecasting  weather,  30,  135,  157. 

Kites,    167,    173,    179,    180;    Har- 
grave,   170,    174,    177;    observar 

Franklin,  Benjamin,  159. 
Fresnel,  142. 

tion,     173;    photography,     174; 
war,  173,  176. 

Frost,  109. 

Kona,  136 

Krakatoa,  140,  146. 

G. 

Galileo,  25,  39. 

L. 

Garnerin,  164. 
Gases,  kinetic  theory  of,  90. 
Gay-Lussac,  96,  164. 
Glaisher,    balloon   ascension,    15, 
41,  139,  164. 
Gravitation,  10,  12. 

Labrador  current,  no. 
Langley,    Prof.    S.     P..    36,    166, 
172,  180. 
La  Place,  nebular  theory,  9. 
Laws,  atmospheric,  94. 

Green,  164. 
Gulf  Stream,  46,  60,  no. 

Lemstrom,  149. 
Leste,  136. 
Leveche,  136. 

H. 

Ley,  Rev.  Clement,   in,  157. 

Hadley,  theory  of  the  winds,  6-(, 

Lichtenberg,  163. 
Light,  102,  141. 

74- 

Lightning.  157,  159. 

Hagstrom,  in. 

Lightning  rods,  122,  160. 

Hail,  106,  119,  120,  157. 

Lilienthal,  166. 

INDEX. 


Looming,  146. 

Poisson's  law,  96,  98. 
Priestley,   Dr.  Joseph,   17. 

Mackerel  sky,  113. 

R. 

Mat  dc  ntontagne,  27. 

Radiation,  31. 

Marriotte's  law,  95. 
Mars,  atmosphere,  n. 

Rain    22,  84,  99,   106,   119,  157. 
Rainbow,  147. 

Marvin,  Professor,  179. 

Rainfall,  122. 

Maury,   theory   of  the  wind,  64, 

Redneld,  128. 

74- 

Reid,  128. 

Maxim,  Hiram,  166,  172. 
Meldrum,  Dr.,  130. 

Reye,  Professor,  131. 
Rotch,  174. 

"  Merry  Dancers,"  148. 

Rozier,  Pilatre  de,  163. 

Meteorites,  15,  16. 

Mirage,  145. 

Mist,  109. 
Mistral,  i;l 
Mock-moons,  145. 
Mock-suns,  145. 

Sailing,  180. 
St.  Elmo's  fire,  161. 
Seasonal  changes,  33. 

Mollcr,  93. 

Simoom,  149. 

Monsoons,  58,  80. 
Montgolfiers,  the,  163. 
Moon,  atmosphere  of  the,  it. 
Mormon  tabernacle,  140. 

Sirocco,  136. 
Smyth,  Prof.  Piazzi,  84. 
Snow,  106,  119,  120. 
Snow  line,  perpetual,  41,  42. 
Solano,  136. 

Sound,    atmosphere   as    the   con- 

N. 

veyer  of,  138. 

Nadar,  164. 
Nebular  theory,  9. 

Southerly   bursters,   136. 
Space,  temperature  of,  13,  32,  62, 

105. 

Nimbus  clouds.     See  Clouds. 

Spectre  of  the  Brocken,  148. 

Nitragin,  bacilli,  20. 

Spectrum,  102,  142. 

Nitrogen  in  atmosphere,  18,  19. 
Nortes,  136. 
Northern  lights,  148. 
Nor'-wester,  150. 

Squalls,  150,  157. 
Stevenson,  T.,  91. 
Storms,  99,  104,  106,  125. 
Storms,  law  of,  128. 
Stratus  clouds.     See  Clouds. 

Q 

Sun,  atmosphere,  9,   12,   14;  heat 

of,  32. 

Ocean  currents,  46,  60,  no. 
Oxygen  in  atmosphere,  18,   19. 

Sunset,  colours  at,  103,  143. 
Sun  spots,  54,  149. 

Ozone  in  atmosphere,  18,  21. 

T. 

p. 

Taifuns,  130. 

Taj  Mahal,  140. 

Pampero,  136,  150. 

Temperature,    31;    variations    in. 

Parachute,  163. 
Paraselenae,  145. 

Thermometers,     39;     self-record- 

Parhelia, 145. 

ing,  40. 

Peri-cyclone,  132. 

Thunder,  140,  160. 

Photography,  174. 

Thunder-storms,   59,   99,   120,   149, 

Piddington,  128. 

156. 

Planets,   origin   and   atmosphere 

Tissandier,  164. 

of,  10. 

Tornadoes,    59,   99,    121,    125,   149, 

Pocock,  173. 

150. 

Poisson,  66. 

Torricelli,  25;  vacuum  of,  26. 

I94 

Trade  winds,  64,  79. 
Trilobites,  13. 
Twilight,  15,  16,  143. 


Vettin,  Dr.,  92,  in. 

Viscosity,  90. 

Von  Bezold,  Dr.,  63,  112. 


Water  in  atmosphere,  22,  27, 

1 06. 
Waterspouts,  149,  152. 


Weather  forecasting.  30,  135,  157. 

Wenham,  166. 

Whirlwinds,  59,  149. 

Whispering  Gallery,  140. 

Windmills,  181. 

Winds,  64;  effect  of  tempera- 
ture, 60;  trade,  64,  79;  velocity 
of,  88. 

Y. 

Young,  142. 


Zero,  determination  of,  39. 
Zurbriggen,  15. 


THE   END. 


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